#include <bits/stdc++.h>
using namespace std;
const long double pi = acos(-1);
const long long inf = 1e18, M = 1e9 + 7;
const long long int N = 1;
bool left_turn(pair<long long int, long long int> a,
               pair<long long int, long long int> b,
               pair<long long int, long long int> c) {
  if (a.first == b.first) {
    if (a.second < b.second)
      return c.first < b.first;
    else
      return c.first > b.first;
  } else if (a.second == b.second) {
    if (a.first < b.first)
      return c.second > b.second;
    else
      return c.second < b.second;
  }
}
void solve() {
  long long int n;
  cin >> n;
  vector<pair<long long int, long long int> > v(n + 1);
  for (long long int i = 0; i < n + 1; ++i) cin >> v[i].first >> v[i].second;
  long long int ans = 0;
  for (long long int i = 1; i < n; ++i) {
    if (left_turn(v[i - 1], v[i], v[i + 1])) ++ans;
  }
  cout << ans;
}
int32_t main() {
  long long int t = 1;
  for (long long int z = 1; z <= t; ++z) {
    solve();
  }
  return 0;
}
