#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
ll n, m, k, ans;
const int N = 4e6 + 5;
const int P = 1e9 + 7;
bool ck(ld x, ld y, ld xx, ld yy) {
    ld k = abs(y) / abs(x);
    ld k1 = abs(yy) / abs(xx);
    if (k < k1) {
        return 1;
    }
    return 0;
}
void solve() {
    cin >> n;
    vector<pii> ve(n + 2);
    for (int i = 0; i < n; i++) {
        cin >> ve[i].first >> ve[i].second;
        // cout << i << ' ' << ve[i].first << ' ' << ve[i].second << '\n';
    }
    for (int i = 0; i < n; i++) {
        int pre = (i + n - 1) % n;

        int j = (i + 1) % n;
        while (ve[i].second == ve[j].second) {
            j = (j + 1) % n;
        }
        if (ve[i].second >= ve[j].second || ve[pre].second <= ve[i].second) {
            continue;
        }
        if (ve[i].second != ve[(i + 1) % n].second) {
            if (ve[i].first < ve[j].first || ve[pre].first < ve[i].first) {
                ans++;
                if (ve[pre].first < ve[i].first && ve[i].first > ve[j].first &&
                    ck(ve[i].first - ve[j].first, ve[j].second,
                       ve[i].first - ve[pre].first, ve[pre].second)) {
                   // cout << pre << ' ' << i << ' ' << j << '\n';
                    ans--;
                } else if (ve[i].first < ve[pre].first &&
                           ve[i].first < ve[j].first &&
                           ck(ve[pre].first - ve[i].first, ve[pre].second,
                              ve[j].first - ve[i].first, ve[j].second)) {
                    // cout << pre << ' ' << i << ' ' << j << '\n';
                    ans--;
                }
            }

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