#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
ll n, m, k, ans;
const int N = 4e6 + 5;
const int P = 1e9 + 7;
void solve() {
    cin >> n;
    vector<pii> ve(n + 2);
    for (int i = 0; i < n; i++) {
        cin >> ve[i].first >> ve[i].second;
    }
    for (int i = 0; i < n; i++) {
        int pre = (i + n - 1) % n;

        if (ve[pre].second <= ve[i].second) {
            continue;
        }
        int j = i + 1;
        while (j != i && ve[i].second == ve[j].second) {
            j = (j + 1) % n;
        }
        if (ve[i].second > ve[j].second) {
            continue;
        }
        if (ve[pre].first <= ve[i].first && ve[i].first <= ve[j].first &&
            ve[i].second < ve[j].second) {
            ans++;
        }
    }
    cout << ans << '\n';
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int _ = 1;
    // cin >> _;
    while (_--) {
        solve();
    }
    return 0;
}