#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int l = 1;
  int r = n;
  int i = 1;
  int cnt = 0;
  while (l < r) {
    if (i % 2 == 1)
      l++;
    else
      r--;
    cnt++;
  }
  cout << cnt / 2;
  return 0;
}
