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>
using namespace std;
int main()
{
  int n,res;
  cin >> n;
  res=(n-1)/2;
  cout << res << endl;
  return 0;
}