How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

* 1 \leq N \leq 10^6
* N is an integer.

Input

Input is given from Standard Input in the following format:


N


Output

Print the answer.

Examples

Input

4


Output

1


Input

999999


Output

499999
#include <iostream>

int main(void){
    int N;
    std::cin >> N;
    std::cout << (N%2 == 0 ? N/2 - 1 : N/2);
    return 0;
}