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