#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int64_t n;
  cin >> n;
  for (int64_t i = 0; i < n + 1; ++i) {
    int64_t x, y;
    cin >> x >> y;
  }
  cout << (n - 4) / 2;
}
