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