#include <bits/stdc++.h>
using namespace std;
vector<long long> v;
int main() {
  int n;
  cin >> n;
  int i, x[n + 1], y[n + 1];
  for (i = 0; i < n + 1; ++i) cin >> x[i] >> y[i];
  cout << (n - 4) / 2;
  return 0;
}
