105. Div 3

time limit per test: 0.25 sec.
memory limit per test: 4096 KB


There is sequence 1, 12, 123, 1234, ..., 12345678910, ... . Given first N elements of that sequence. You must determine amount of numbers in it that are divisible by 3.


Input

Input contains N (1<=N<=231 - 1).


Output

Write answer to the output.


Sample Input

4
Sample Output

2
