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