<|instruction|>
Solve the below problem. The solution should start with an abridged problem statement. Then key observations. Then full solution based on the observations. Then C++ and Python implementations with comments.

528. Bencoding
Time limit per test: 0.25 second(s)
Memory limit: 262144 kilobytes
input: standard
output: standard

Bencoding is a modern technique which is used for representing data structures as sequences of characters. It it capable of encoding strings, integers, lists and dictionaries as specified below:


every string s is encoded as "<k>:<s>", where "<s>" is the string itself and "<k>" is its length written with no leading zeros (with the exception of integer zero, which is always represented as "0"). Bencoding is capable of encoding empty strings, so s is allowed to be empty.

For example, "4:spam" represents the string "spam", "0:" represents an empty string.

every integer n is encoded as "i<n>e", where "<n>" is the number itself written with no leading zeros (with the exception of integer zero, which is always represented as "0"). Bencoding is capable of encoding very large integers, so n doesn't necessarily fit into any of the standard integer data types provided by programming languages. Only non-negative integers can be encoded using bencoding.

For example, "i1024e" represents the number 1024.

every list items containing n elements item0, item1,..., itemn-1 is encoded as "l<item0><item1> <itemn-1>e". Each item of the list itemi is a supported data structure encoded using bencoding technique. Lists are allowed to be empty.

For example, "li101el4:spami1024eee" represents the list "[ 101, [ "spam", 1024 ] ]".

every dictionary dict consisting of n keys key0, key1,..., keyn-1 mapped into n values value0, value1,..., valuen-1 correspondingly is encoded as "d<key0><value0><key1><value1> <keyn-1><valuen-1>e". All keys and values are supported data structures encoded using bencoding technique. A dictionary may have duplicate keys and/or values. Dictionaries are allowed to be empty.

For example, "d1:a0:1:pl1:b2:cdee" represents the dictionary with string key "a" mapped into an empty string "", and key "p" mapped into the list "[ "b", "cd" ]".


A character sequence c is called a  if the following two conditions are met:
c is a correct bencoded representation of a single string, integer, list or dictionary;
the number of characters in c doesn't exceed a given number m.


For example, when m=3, the sequence c="2:bc" is not considered a valid bencoded object even though it represents a correctly encoded string "bc".

Given m and c you have to write a program which should determine whether c is a . If c is not a , it also has to find the longest prefix of c which could be a prefix of some . Formally, you should find a maximal position j within the given character sequence c, such that a prefix of c up to, but not including, character at position j could be a prefix of some . If the given character sequence c is not a , but the entire sequence c is a prefix of some , then j is considered equal to the length of c.

Input
The first line of the input contains one integer m (2 ≤ m ≤ 109)  — the maximum possible length of a valid bencoded object. The second line contains a character sequence which you are to process. The sequence will only contain characters with ASCII codes from 33 to 127, inclusive. Its length will be between 1 and 106 characters.

Output
Print a single line containing word "ok" (without quotes) if the given input character sequence is a valid bencoded object. Otherwise, print message "Error at position j!". The first character of the input sequence is considered to have position "0".

Example(s)
sample input
sample output
14
li10e11:abcdefghijke
Error at position 6!

sample input
sample output
10
i-1e
Error at position 1!

sample input
sample output
3
i2
Error at position 2!

sample input
sample output
18
dli1eei1ei1eli1eee
ok



Note
In the first sample test the given sequence is not a valid bencoded object. But its prefix "li10e1" can be extended to a valid bencoded object while not exceeding 14 characters in length (for example, "li10e1:xe"). It's not the case with longer prefixes of length 7 and more, so j=6 in this case.

<|response|>
## 1. Abridged Problem Statement

Bencoding can encode four object types:

- **String**: `k:s`, where `k` is the string length, no leading zeros except `0`.
- **Integer**: `i<n>e`, where `n` is a non-negative integer, no leading zeros except `0`.
- **List**: `l<item1><item2>...e`
- **Dictionary**: `d<key1><value1><key2><value2>...e`

Lists and dictionaries may be empty. Dictionary keys and values can be any bencoded object.

Given:

- maximum allowed encoded length `m`,
- a character sequence `c`,

determine whether `c` is exactly one valid bencoded object of length at most `m`.

If yes, print:

```text
ok
```

Otherwise, find the maximum prefix length `j` such that `c[0:j]` can still be extended into some valid bencoded object whose total length is at most `m`, and print:

```text
Error at position j!
```

Positions are zero-based, and `j` is also the prefix length.

---

## 2. Key Observations

### Observation 1: We can parse left to right

We only need one scan over the input string.

While scanning, we maintain:

1. A stack of currently opened containers.
2. The current parsing state: are we reading an integer, a string length, a string body, or are we at an item boundary?

---

### Observation 2: Containers need state

Lists are simple: inside a list, we may either read another item or close the list with `e`.

Dictionaries are more complicated. A dictionary alternates between keys and values:

- expecting a key, or possibly closing with `e`,
- expecting a value, where closing with `e` is illegal.

So stack frames can be:

```text
LIST
DICT_KEY     dictionary expects next key or closing e
DICT_VALUE   dictionary expects a value
```

---

### Observation 3: Syntax correctness is not enough

A prefix may be syntactically valid but already impossible to complete within length `m`.

Example:

```text
m = 3
c = 2:bc
```

The full string `2:bc` is syntactically a valid bencoded string, but its length is `4`, which exceeds `m`.

So after each consumed character, we must ask:

```text
Can the current prefix still be completed within m characters?
```

---

### Observation 4: We need minimum remaining length

For every valid prefix, compute the minimum number of extra characters needed to finish some valid object.

If:

```text
current_prefix_length + minimum_remaining_length > m
```

then this prefix can no longer be extended to a valid object within the length limit.

---

### Observation 5: Minimum completion can be maintained in O(1)

The smallest complete object has length `2`, for example:

```text
0:
le
de
```

For open containers, we maintain the sum of their future closing costs.

Define `g_cost(frame)` as the minimum cost to finish this container after the currently active item inside it is completed:

```text
g_cost(LIST)       = 1   close list with e
g_cost(DICT_KEY)   = 3   after key, need shortest value 0: and closing e
g_cost(DICT_VALUE) = 1   after value, can close dictionary with e
```

Maintain:

```text
g_sum = sum of g_cost(frame) over all open frames
```

Also define `f_cost(frame)` as the minimum cost to finish a container when we are currently at a clean boundary inside it:

```text
f_cost(LIST)       = 1   close list with e
f_cost(DICT_KEY)   = 1   close dictionary with e
f_cost(DICT_VALUE) = 3   need shortest value 0: and closing e
```

This allows calculating the minimum remaining length in constant time.

---

## 3. Full Solution Approach

We scan the input string from left to right.

### Parser states

We use four parsing states:

```text
NONE       currently at a boundary between objects
INT        currently reading an integer after i
STR_LEN    currently reading string length digits
STR_BODY   currently reading string body characters
```

For integers we track:

- number of digits read,
- whether the first digit was `0`.

For string lengths we track:

- parsed length,
- number of length digits read,
- whether the first digit was `0`.

For string body we track:

- how many body characters remain.

---

### Starting a new item

At an item boundary, valid starting characters are:

```text
i       starts integer
l       starts list
d       starts dictionary
0-9     starts string length
```

Any other character is invalid.

---

### Completing an item

Whenever a full item is completed:

- If there is no open container, then the root object is complete.
- If parent is `LIST`, nothing changes.
- If parent is `DICT_KEY`, then the dictionary now expects a value.
- If parent is `DICT_VALUE`, then the dictionary now expects another key or closing `e`.

---

### Closing a container

At a boundary inside:

- `LIST`: `e` closes the list.
- `DICT_KEY`: `e` closes the dictionary.
- `DICT_VALUE`: `e` is invalid because a value is required.

When a list or dictionary closes, that whole container becomes a completed item, so we call the same item-completion logic.

---

### Minimum remaining length

There are two main cases.

#### Case 1: At a clean boundary

If no root object has started yet:

```text
minimum_remaining = 2
```

because the shortest object is `0:`, `le`, or `de`.

If root is already complete:

```text
minimum_remaining = 0
```

If we are inside some container:

```text
minimum_remaining = f_cost(top_frame) + g_sum - g_cost(top_frame)
```

We finish the innermost frame from a boundary using `f_cost`, and all outer frames using their `g_cost`.

---

#### Case 2: In the middle of an atomic item

If reading an integer:

```text
if no digits yet:
    need one digit and e => 2
else:
    need e => 1
```

If reading string length:

```text
need ':' plus exactly length body characters
```

If reading string body:

```text
need remaining body characters
```

After finishing the current item, all open containers still need to be completed, so add `g_sum`.

---

### Tracking the answer

Let `best` be the largest feasible prefix length found so far.

After processing each character:

1. If syntax becomes invalid, stop.
2. Compute minimum remaining length.
3. If current length plus minimum remaining length exceeds `m`, stop.
4. Otherwise update `best`.

At the end:

- If the whole string was consumed and the root object is complete, print `ok`.
- Otherwise print the error at `best`.

Complexities:

```text
Time:  O(n)
Memory: O(n)
```

where `n` is the length of the input string.

---

## 4. C++ Implementation with Detailed Comments

```cpp
#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    long long m;
    string c;

    cin >> m >> c;

    /*
        Container stack frame types.

        LIST:
            We are inside a list. At a boundary, we may read another item
            or close the list with 'e'.

        DICT_KEY:
            We are inside a dictionary and expect a key, or we may close
            the dictionary with 'e'.

        DICT_VALUE:
            We are inside a dictionary and must read a value.
            Closing with 'e' is not allowed here.
    */
    enum Frame {
        LIST,
        DICT_KEY,
        DICT_VALUE
    };

    /*
        Current parser state.
    */
    enum Progress {
        NONE,       // At a clean item boundary
        INT,        // Reading integer digits after 'i'
        STR_LEN,    // Reading string length digits
        STR_BODY    // Reading string body
    };

    vector<Frame> stack_frames;

    Progress progress = NONE;

    // Integer parsing data.
    int int_digits = 0;
    bool int_started_with_zero = false;

    // String parsing data.
    // In STR_LEN, str_value is the parsed length.
    // In STR_BODY, str_value is remaining body characters.
    long long str_value = 0;
    int str_len_digits = 0;
    bool str_len_started_with_zero = false;

    bool root_started = false;
    bool root_completed = false;

    /*
        g_sum is the sum of g_cost over all open containers.

        g_cost means:
        Minimum number of characters needed to complete this frame after
        the currently active item inside it has been completed.
    */
    long long g_sum = 0;

    auto g_cost = [](Frame frame) -> int {
        if (frame == DICT_KEY) {
            /*
                If a key has just completed, the dictionary still needs
                a value. The shortest value is "0:" and then we close
                the dictionary with 'e'.

                Total: "0:e", length 3.
            */
            return 3;
        }

        /*
            LIST:
                after an item, close with 'e'.

            DICT_VALUE:
                after a value, close with 'e'.
        */
        return 1;
    };

    auto f_cost = [](Frame frame) -> int {
        if (frame == DICT_VALUE) {
            /*
                At a clean boundary in a dictionary that expects a value,
                we must add a shortest value "0:" and then close with 'e'.

                Total length 3.
            */
            return 3;
        }

        /*
            LIST:
                close with 'e'.

            DICT_KEY:
                close dictionary with 'e'.
        */
        return 1;
    };

    /*
        To avoid overflow while reading very long string lengths, we cap
        parsed lengths. Since m <= 1e9, any length greater than m already
        makes completion impossible. m + 1 is enough as a sentinel.
    */
    const long long CAP = m + 1;

    auto min_remaining = [&]() -> long long {
        /*
            If the root object is already complete, no more characters
            are needed.
        */
        if (root_completed) {
            return 0;
        }

        /*
            If we are not in the middle of an integer or string.
        */
        if (progress == NONE) {
            /*
                No object has started yet. The shortest valid object
                has length 2: "0:", "le", or "de".
            */
            if (!root_started) {
                return 2;
            }

            /*
                If no frames are open, then normally the root is complete.
                This case is mostly defensive.
            */
            if (stack_frames.empty()) {
                return 0;
            }

            /*
                We are at a boundary inside the top frame.

                Finish the top frame using f_cost, and finish all outer
                frames using their g_cost. g_sum contains the cost for
                all frames, so remove the top frame's g_cost and replace
                it with f_cost.
            */
            Frame top = stack_frames.back();
            return f_cost(top) + (g_sum - g_cost(top));
        }

        /*
            Otherwise, we are in the middle of an atomic object.
        */
        long long finish_current_item = 0;

        if (progress == INT) {
            /*
                Integer needs at least one digit and a closing 'e'.

                If no digit has been read yet:
                    need one digit + 'e' => 2
                Otherwise:
                    need only 'e' => 1
            */
            finish_current_item = (int_digits == 0 ? 1 : 0) + 1;
        } else if (progress == STR_LEN) {
            /*
                Need ':' plus exactly str_value body characters.
            */
            finish_current_item = 1 + str_value;
        } else {
            /*
                STR_BODY: need remaining body characters.
            */
            finish_current_item = str_value;
        }

        /*
            After the current item finishes, all open containers need to
            be completed.
        */
        return finish_current_item + g_sum;
    };

    /*
        Called whenever one complete bencoded item has just been parsed.
    */
    auto item_completed = [&]() {
        if (stack_frames.empty()) {
            /*
                A complete item outside any container is the root object.
            */
            root_completed = true;
            return;
        }

        Frame &top = stack_frames.back();

        if (top == DICT_KEY) {
            /*
                A dictionary key was just completed.
                Now the dictionary expects a value.
            */
            g_sum -= g_cost(DICT_KEY);
            top = DICT_VALUE;
            g_sum += g_cost(DICT_VALUE);
        } else if (top == DICT_VALUE) {
            /*
                A dictionary value was just completed.
                Now the dictionary expects another key or closing 'e'.
            */
            g_sum -= g_cost(DICT_VALUE);
            top = DICT_KEY;
            g_sum += g_cost(DICT_KEY);
        }

        /*
            If top is LIST, nothing changes.
        */
    };

    /*
        Try to start a new bencoded item using character ch.
        Returns false if ch cannot start any item.
    */
    auto start_item = [&](char ch) -> bool {
        if (ch == 'i') {
            progress = INT;
            int_digits = 0;
            int_started_with_zero = false;
            return true;
        }

        if (ch == 'l') {
            stack_frames.push_back(LIST);
            g_sum += g_cost(LIST);
            return true;
        }

        if (ch == 'd') {
            stack_frames.push_back(DICT_KEY);
            g_sum += g_cost(DICT_KEY);
            return true;
        }

        if ('0' <= ch && ch <= '9') {
            progress = STR_LEN;
            str_value = ch - '0';
            str_len_digits = 1;
            str_len_started_with_zero = (ch == '0');
            return true;
        }

        return false;
    };

    /*
        Process one character.
        Returns false if the prefix becomes syntactically impossible.
    */
    auto process_char = [&](char ch) -> bool {
        /*
            No characters are allowed after the root object has completed.
        */
        if (root_completed) {
            return false;
        }

        if (progress == INT) {
            if ('0' <= ch && ch <= '9') {
                /*
                    Leading zero is illegal unless the integer is exactly 0.
                    Therefore, after reading first digit '0', no more digits
                    may follow.
                */
                if (int_started_with_zero && int_digits >= 1) {
                    return false;
                }

                if (ch == '0' && int_digits == 0) {
                    int_started_with_zero = true;
                }

                int_digits++;
                return true;
            }

            if (ch == 'e') {
                /*
                    Integer must contain at least one digit.
                    "ie" is invalid.
                */
                if (int_digits == 0) {
                    return false;
                }

                progress = NONE;
                item_completed();
                return true;
            }

            return false;
        }

        if (progress == STR_LEN) {
            if ('0' <= ch && ch <= '9') {
                /*
                    Leading zero in a string length is illegal unless
                    the length is exactly 0.
                    So "03:abc" is invalid.
                */
                if (str_len_started_with_zero && str_len_digits >= 1) {
                    return false;
                }

                /*
                    Accumulate with a cap to avoid overflow.
                */
                if (str_value <= CAP) {
                    str_value = str_value * 10 + (ch - '0');
                    if (str_value > CAP) {
                        str_value = CAP;
                    }
                }

                str_len_digits++;
                return true;
            }

            if (ch == ':') {
                /*
                    Length is complete, now read the raw body.
                */
                progress = STR_BODY;

                /*
                    Empty string completes immediately.
                */
                if (str_value == 0) {
                    progress = NONE;
                    item_completed();
                }

                return true;
            }

            return false;
        }

        if (progress == STR_BODY) {
            /*
                Any character is allowed inside string body.
            */
            str_value--;

            if (str_value == 0) {
                progress = NONE;
                item_completed();
            }

            return true;
        }

        /*
            progress == NONE:
            We are at a clean boundary.
        */

        if (stack_frames.empty()) {
            /*
                If the root already started and we are back outside all
                containers, then another item would be trailing garbage.
            */
            if (root_started) {
                return false;
            }

            root_started = true;
            return start_item(ch);
        }

        Frame top = stack_frames.back();

        if (top == LIST || top == DICT_KEY) {
            /*
                Lists and dictionaries expecting keys may close with 'e'.
            */
            if (ch == 'e') {
                g_sum -= g_cost(top);
                stack_frames.pop_back();

                /*
                    The closed list/dictionary is itself a completed item.
                */
                item_completed();
                return true;
            }

            /*
                Otherwise, start a nested item.
            */
            return start_item(ch);
        }

        /*
            top == DICT_VALUE:
            We must read a value. Closing with 'e' is illegal.
        */
        if (ch == 'e') {
            return false;
        }

        return start_item(ch);
    };

    int n = (int)c.size();

    /*
        best is the largest prefix length that is syntactically valid so far
        and can still be completed within total length m.
    */
    int best = 0;

    for (int i = 0; i < n; i++) {
        if (!process_char(c[i])) {
            break;
        }

        long long prefix_length = i + 1;

        /*
            Check length feasibility.
        */
        if (prefix_length + min_remaining() > m) {
            break;
        }

        best = (int)prefix_length;
    }

    if (best == n && root_completed) {
        cout << "ok\n";
    } else {
        cout << "Error at position " << best << "!\n";
    }

    return 0;
}
```

---

## 5. Python Implementation with Detailed Comments

```python
import sys


def solve() -> None:
    data = sys.stdin.read().splitlines()

    m = int(data[0])
    c = data[1]

    /*
    Python does not support C-style comments.
    The implementation below uses normal Python comments.
    */

if __name__ == "__main__":
    solve()
```

The above skeleton contains an invalid Python comment block, so here is the complete correct Python implementation:

```python
import sys


def solve() -> None:
    data = sys.stdin.read().splitlines()

    m = int(data[0])
    c = data[1]

    # Container frame types.
    LIST = 0
    DICT_KEY = 1
    DICT_VALUE = 2

    # Parser progress states.
    NONE = 0
    INT = 1
    STR_LEN = 2
    STR_BODY = 3

    stack_frames = []

    progress = NONE

    # Integer parsing data.
    int_digits = 0
    int_started_with_zero = False

    # String parsing data.
    # In STR_LEN, str_value stores parsed length.
    # In STR_BODY, str_value stores remaining body characters.
    str_value = 0
    str_len_digits = 0
    str_len_started_with_zero = False

    root_started = False
    root_completed = False

    # Sum of g_cost over all currently open frames.
    g_sum = 0

    # Cap large string lengths to avoid huge unnecessary integers.
    # Since m <= 1e9, any length above m is already too large.
    CAP = m + 1

    def g_cost(frame: int) -> int:
        """
        Minimum cost to complete this frame after the currently active
        item inside it finishes.
        """
        if frame == DICT_KEY:
            # After a key finishes, need shortest value "0:" and closing 'e'.
            return 3

        # LIST or DICT_VALUE can be closed with one 'e' after the item.
        return 1

    def f_cost(frame: int) -> int:
        """
        Minimum cost to complete this frame from a clean boundary inside it.
        """
        if frame == DICT_VALUE:
            # Must provide shortest value "0:" and then close with 'e'.
            return 3

        # LIST or DICT_KEY can close immediately with 'e'.
        return 1

    def min_remaining() -> int:
        """
        Compute the minimum number of extra characters needed to finish
        the current prefix as a valid bencoded object.
        """
        nonlocal progress
        nonlocal root_started, root_completed
        nonlocal str_value, int_digits
        nonlocal g_sum

        if root_completed:
            return 0

        if progress == NONE:
            if not root_started:
                # Shortest object is "0:", "le", or "de".
                return 2

            if not stack_frames:
                return 0

            top = stack_frames[-1]

            # Replace top frame's g_cost by f_cost.
            return f_cost(top) + (g_sum - g_cost(top))

        if progress == INT:
            # Need one digit if none read yet, then final 'e'.
            finish_current_item = (1 if int_digits == 0 else 0) + 1

        elif progress == STR_LEN:
            # Need ':' and then str_value body characters.
            finish_current_item = 1 + str_value

        else:
            # STR_BODY: need remaining body characters.
            finish_current_item = str_value

        # After current item finishes, all containers must be completed.
        return finish_current_item + g_sum

    def item_completed() -> None:
        """
        Called when a full item has just been parsed.
        Updates parent container state or marks the root as complete.
        """
        nonlocal root_completed, g_sum

        if not stack_frames:
            root_completed = True
            return

        top = stack_frames[-1]

        if top == DICT_KEY:
            # Key completed; dictionary now expects value.
            g_sum -= g_cost(DICT_KEY)
            stack_frames[-1] = DICT_VALUE
            g_sum += g_cost(DICT_VALUE)

        elif top == DICT_VALUE:
            # Value completed; dictionary now expects next key or closing 'e'.
            g_sum -= g_cost(DICT_VALUE)
            stack_frames[-1] = DICT_KEY
            g_sum += g_cost(DICT_KEY)

        # LIST does not change.

    def start_item(ch: str) -> bool:
        """
        Try to start a new bencoded item with character ch.
        Return False if ch cannot start any valid item.
        """
        nonlocal progress
        nonlocal int_digits, int_started_with_zero
        nonlocal str_value, str_len_digits, str_len_started_with_zero
        nonlocal g_sum

        if ch == 'i':
            progress = INT
            int_digits = 0
            int_started_with_zero = False
            return True

        if ch == 'l':
            stack_frames.append(LIST)
            g_sum += g_cost(LIST)
            return True

        if ch == 'd':
            stack_frames.append(DICT_KEY)
            g_sum += g_cost(DICT_KEY)
            return True

        if '0' <= ch <= '9':
            progress = STR_LEN
            str_value = ord(ch) - ord('0')
            str_len_digits = 1
            str_len_started_with_zero = (ch == '0')
            return True

        return False

    def process_char(ch: str) -> bool:
        """
        Process one character.
        Return False if this character makes the prefix syntactically invalid.
        """
        nonlocal progress
        nonlocal int_digits, int_started_with_zero
        nonlocal str_value, str_len_digits, str_len_started_with_zero
        nonlocal root_started, root_completed
        nonlocal g_sum

        # No extra characters are allowed after the root object is complete.
        if root_completed:
            return False

        if progress == INT:
            if '0' <= ch <= '9':
                # Leading zero is illegal unless the integer is exactly 0.
                if int_started_with_zero and int_digits >= 1:
                    return False

                if ch == '0' and int_digits == 0:
                    int_started_with_zero = True

                int_digits += 1
                return True

            if ch == 'e':
                # Integer must contain at least one digit.
                if int_digits == 0:
                    return False

                progress = NONE
                item_completed()
                return True

            return False

        if progress == STR_LEN:
            if '0' <= ch <= '9':
                # Leading zero in string length is illegal unless length is 0.
                if str_len_started_with_zero and str_len_digits >= 1:
                    return False

                # Accumulate with cap.
                if str_value <= CAP:
                    str_value = str_value * 10 + (ord(ch) - ord('0'))
                    if str_value > CAP:
                        str_value = CAP

                str_len_digits += 1
                return True

            if ch == ':':
                progress = STR_BODY

                # Empty string completes immediately.
                if str_value == 0:
                    progress = NONE
                    item_completed()

                return True

            return False

        if progress == STR_BODY:
            # Any character can appear inside a string body.
            str_value -= 1

            if str_value == 0:
                progress = NONE
                item_completed()

            return True

        # progress == NONE: we are at an item boundary.

        if not stack_frames:
            # If root already started, another item means trailing garbage.
            if root_started:
                return False

            root_started = True
            return start_item(ch)

        top = stack_frames[-1]

        if top == LIST or top == DICT_KEY:
            # Lists and dictionaries expecting keys may close with 'e'.
            if ch == 'e':
                g_sum -= g_cost(top)
                stack_frames.pop()

                # Closed list/dictionary is itself a completed item.
                item_completed()
                return True

            # Otherwise start a nested item.
            return start_item(ch)

        # top == DICT_VALUE: must read a value, cannot close.
        if ch == 'e':
            return False

        return start_item(ch)

    best = 0

    for i, ch in enumerate(c):
        if not process_char(ch):
            break

        prefix_length = i + 1

        if prefix_length + min_remaining() > m:
            break

        best = prefix_length

    if best == len(c) and root_completed:
        print("ok")
    else:
        print(f"Error at position {best}!")


if __name__ == "__main__":
    solve()
```