#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:500000000")
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n % 2 == 0) {
    cout << n / 2 - 1 << endl;
  } else {
    cout << n / 2 << endl;
  }
}
