#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, ans = 0, l, r;
  cin >> n;
  l = 1;
  r = n;
  while (true) {
    l++;
    r--;
    if (l > r) break;
    ans++;
  }
  cout << ans << endl;
  return 0;
}
