#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e5 + 5;
struct node {
  double x, y;
} a[MAXN];
int main() {
  int n;
  while (scanf("%d", &n) != EOF) {
    for (int i = 0; i <= n; i++) scanf("%d %d", &a[i].x, &a[i].y);
    printf("%d\n", (n - 4) / 2);
  }
  return 0;
}
