#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, ans;
  int x[1055];
  int y[1055];
  while (~scanf("%d", &n)) {
    for (int i = 0; i <= n; i++) {
      scanf("%d%d", &x[i], &y[i]);
    }
    ans = (n - 4) / 2;
    printf("%d\n", ans);
  }
  return 0;
}
