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