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