#include <bits/stdc++.h>
using namespace std;
const int MXN = 1e5 + 1;
const int INF = 1e9 + 7;
long long n;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    int x, y;
    cin >> x >> y;
  }
  cout << n / 2 - 2;
  return 0;
}
