#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 9;
const long long INF = 2e9 + 9;
const double PI = acos(-1.0);
const double eps = 1e-3;
int n;
int main() {
  double x, y;
  for (; ~scanf("%d", &n);) {
    for (int i = 0; i <= n; i++) {
      scanf("%lf%lf", &x, &y);
    }
    printf("%d\n", (n - 4) / 2);
  }
  return 0;
}
