p317.ans1
======================
6.30000000

=================
p317.in1
======================
10 3
5 2
1 100
10 3
0 1
1 50
8 1
2 3

=================
p317.cpp
======================
#include <bits/stdc++.h>
// #include <coding_library/dp_optimizations/convex_hull_trick.hpp>

using namespace std;

template<typename T1, typename T2>
ostream& operator<<(ostream& out, const pair<T1, T2>& x) {
    return out << x.first << ' ' << x.second;
}

template<typename T1, typename T2>
istream& operator>>(istream& in, pair<T1, T2>& x) {
    return in >> x.first >> x.second;
}

template<typename T>
istream& operator>>(istream& in, vector<T>& a) {
    for(auto& x: a) {
        in >> x;
    }
    return in;
};

template<typename T>
ostream& operator<<(ostream& out, const vector<T>& a) {
    for(auto x: a) {
        out << x << ' ';
    }
    return out;
};

template<class T, bool maximum = true>
class ConvexHullTrick {
  public:
    static constexpr T inf = is_integral_v<T> ? numeric_limits<T>::max()
                                              : numeric_limits<T>::infinity();

    T div(T a, T b) {
        if constexpr(is_integral_v<T>) {
            return a / b - ((a ^ b) < 0 && a % b);
        } else {
            return a / b;
        }
    }

    struct Line {
        mutable T k, m, p;
        bool operator<(const Line& o) const { return k < o.k; }
        bool operator<(T x) const { return p < x; }
    };

    multiset<Line, less<>> hull;

    using Iter = typename multiset<Line, less<>>::iterator;

    bool intersect(Iter x, Iter y) {
        if(y == hull.end()) {
            return x->p = inf, 0;
        }
        if(x->k == y->k) {
            x->p = x->m > y->m ? inf : -inf;
        } else {
            x->p = div(y->m - x->m, x->k - y->k);
        }
        return x->p >= y->p;
    }

    void add(T k, T m) {
        if constexpr(!maximum) {
            k = -k;
            m = -m;
        }
        auto z = hull.insert({k, m, 0}), y = z++, x = y;
        while(intersect(y, z)) {
            z = hull.erase(z);
        }
        if(x != hull.begin() && intersect(--x, y)) {
            intersect(x, y = hull.erase(y));
        }
        while((y = x) != hull.begin() && (--x)->p >= y->p) {
            intersect(x, hull.erase(y));
        }
    }

    T query(T x) {
        assert(!hull.empty());
        auto l = *hull.lower_bound(x);
        T res = l.k * x + l.m;
        if constexpr(!maximum) {
            res = -res;
        }
        return res;
    }
};

long long B;
int N;

struct Stable {
    long long x;
    vector<pair<long long, long long>> horses;
};

struct SegTree {
    int n;
    vector<ConvexHullTrick<double, false>> tree;

    SegTree() : n(0) {}
    SegTree(int n) : n(n), tree(4 * n) {}

    void update(int ql, int qr, double k, double m, int idx, int lo, int hi) {
        if(ql > hi || qr < lo) {
            return;
        }
        if(ql <= lo && hi <= qr) {
            tree[idx].add(k, m);
            return;
        }
        int mid = (lo + hi) / 2;
        update(ql, qr, k, m, 2 * idx, lo, mid);
        update(ql, qr, k, m, 2 * idx + 1, mid + 1, hi);
    }

    double query(int pos, double x, int idx, int lo, int hi) {
        double res = 1e18;
        if(!tree[idx].hull.empty()) {
            res = tree[idx].query(x);
        }
        if(lo == hi) {
            return res;
        }
        int mid = (lo + hi) / 2;
        if(pos <= mid) {
            return min(res, query(pos, x, 2 * idx, lo, mid));
        } else {
            return min(res, query(pos, x, 2 * idx + 1, mid + 1, hi));
        }
    }
};

vector<Stable> stables;

void read() {
    cin >> B >> N;
    stables.resize(N);
    for(int i = 0; i < N; i++) {
        int m;
        cin >> stables[i].x >> m;
        stables[i].horses.resize(m);
        for(int j = 0; j < m; j++) {
            long long v, d;
            cin >> v >> d;
            stables[i].horses[j] = {d, v};
        }
    }
}

void solve() {
    // We should ideally aim for something that is quicker than O(N * M) which
    // can be done with a simple DP but might be a bit too tight to pass. We
    // know that every horse can get us from some Xi to some other Xj and all
    // stables between these (we can do a binary search to find the relevant
    // ranges). Afterwards, if these two are [i;r], we would like to update
    // every j > i in this using DP[j] := min(DP[j], (Xj - Xi) / vk + DP[j]) for
    // horse k that can get us from i to j. Implementing this naively by going
    // through all horses (M) and the O(N) reachable stables, results in O(N *
    // M).
    //
    // To make this quicker, we can notice that the second term in the minimum
    // above is actually a linear function. This should immediately lead us to
    // thinking about convex hull trick. Let us build a segment tree like
    // structure over the N stables, with a convex hull trick container in each
    // node. Then after processing some stable i, we would like to add Mi lines
    // to various ranges [i+1, rk]. We can decompose these ranges recursively
    // into O(log N) intervals, and then add line individually to each of the
    // containers covered. This update ends up being in O(log N * log M). The
    // query is then done whenever we are processing some stable, and is simply
    // going through the nodes from the root until a leaf, which is again the
    // same complexity of O(log N * log M). We have O(M) updates and queries,
    // which overall is quicker than O(N * M).

    sort(stables.begin(), stables.end(), [](const Stable& a, const Stable& b) {
        return a.x < b.x;
    });

    while(!stables.empty() && stables.back().x > B) {
        stables.pop_back();
    }
    N = stables.size();

    if(N == 0 || stables[0].x != 0) {
        cout << -1 << endl;
        return;
    }

    SegTree seg(N);
    vector<double> dp(N, 1e18);
    double ans = 1e18;

    for(int i = 0; i < N; i++) {
        if(stables[i].x == 0) {
            dp[i] = 0;
        }

        if(i > 0) {
            double q = seg.query(i, (double)stables[i].x, 1, 0, N - 1);
            dp[i] = min(dp[i], q);
        }

        if(dp[i] >= 1e18) {
            continue;
        }

        for(auto& [d, v]: stables[i].horses) {
            long long reach = stables[i].x + d;

            if(reach >= B) {
                ans = min(ans, dp[i] + (double)(B - stables[i].x) / v);
            }

            int r = (int)(upper_bound(
                              stables.begin(), stables.end(), reach,
                              [](long long val, const Stable& s) {
                                  return val < s.x;
                              }
                          ) -
                          stables.begin()) -
                    1;

            if(r > i) {
                double slope = 1.0 / v;
                double intercept = dp[i] - (double)stables[i].x / v;
                seg.update(i + 1, r, slope, intercept, 1, 0, N - 1);
            }
        }
    }

    if(ans >= 1e18) {
        cout << -1 << endl;
    } else {
        cout << fixed << setprecision(8) << ans << endl;
    }
}

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

    int T = 1;
    // cin >> T;
    for(int test = 1; test <= T; test++) {
        read();
        // cout << "Case #" << test << ": ";
        solve();
    }

    return 0;
}

=================
statement.txt
======================
317. Fast Ride
Time limit per test: 0.25 second(s)
Memory limit: 65536 kilobytes
input: standard
output: standard



Is there a Russian who does not like a fast ride?! Royal messenger should get from point A to point B as quickly as possible. Point A has coordinate 0 on the Ox axis. Point B has positive coordinate B on the Ox axis. Of course, the messenger will use horses to get to the destination point. The messenger is not allowed to travel by foot because of an importance of his mission and security reasons.

There are N stables on the straight line from A to B. The i-th stable has Mi horses inside. Each horse is characterized by its speed vj and maximum distance it can gallop dj (after that it falls exhausted). The messenger can change the horse reaching or passing a stable to any horse from that stable without losing time.

Your task is to find the minimum time the messenger needs to get to the point B.

Input
The first line of the input contains two integer numbers B and N (1 ≤ B ≤ 108; 1 ≤ N ≤ 5000), where N is the number of stables. Descriptions of the stables follow. The first line of each description contains two integer numbers Xi, Mi (0 ≤ Xi ≤ 108; 0 < Mi) — the position of the stable and the number of horses in this stable respectively. The following Mi lines contain the description of the horses. Each description of a horse is a pair of integer numbers vj, dj (1 ≤ vj ≤ 108; 1 ≤ dj ≤ 108). The total number of horses in all stables is no more than 105. It is possible that two or more stables have the same position.

Output
Write the minimum time which will take the messenger to get to the point B to the output. The answer should be written with no less than 3 digits after the decimal point. If the solution does not exist, write -1 to the output.

Example(s)
sample input
sample output
10 3
5 2
1 100
10 3
0 1
1 50
8 1
2 3
6.30000000

=================
