#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimization("O3")
#pragma GCC optimization("unroll-loops")
using namespace std;
const bool dbg_flg = 0;
const int inf = 1e9 + 7;
const int MAXN = 1e5 + 5;
int n;
void solve() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    int x, y;
    cin >> x >> y;
  }
  cout << (n - 4) / 2;
}
int main() {
  ios_base::sync_with_stdio(NULL);
  cin.tie(NULL);
  cout.tie(NULL);
  int tt = 1;
  while (tt--) {
    solve();
  }
  return 0;
}
