#include <bits/stdc++.h>
using namespace std;
pair<int, int> pts[1001];
int n;
int main() {
  cin >> n;
  for (int i = 0; i <= n; i++) {
    scanf("%d%d", &pts[i].first, &pts[i].second);
  }
  int ans = 0;
  char last = 'U';
  for (int i = 1; i < n; i++) {
    char cur;
    if (pts[i].first == pts[i + 1].first) {
      if (pts[i].second > pts[i + 1].second) {
        cur = 'D';
      } else {
        cur = 'U';
      }
    } else if (pts[i].second == pts[i + 1].second) {
      if (pts[i].first > pts[i + 1].first) {
        cur = 'L';
      } else {
        cur = 'R';
      }
    }
    if (last == 'R' && cur == 'U')
      ++ans;
    else if (last == 'U' && cur == 'L')
      ++ans;
    else if (last == 'L' && cur == 'D')
      ++ans;
    else if (last == 'D' && cur == 'R')
      ++ans;
    last = cur;
  }
  cout << ans << endl;
  return 0;
}
