#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n > 1 && n % 2 == 0)
    cout << n / 2 - 1;
  else if (n > 1 && n % 2 == 1)
    cout << n / 2;
  else
    cout << 0;
}
