#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e18;
const double eps = 1e-8;
int main() {
  int n;
  cin >> n;
  if (n % 2 == 0) {
    cout << n / 2 - 1 << endl;
  } else {
    cout << n / 2 << endl;
  }
}
