// Coded by hjxddl
#include <bits/stdc++.h>
#define ll long long
#define db double
const int N = 2e5 + 5;
int n;
int ck(int k, std::pair<ll, ll> (&a)[]) {
    int cnt = 0;
    auto [x, y] = a[k];
    for (int i = 1; i <= n; i++) {
        auto [x1, y1] = a[i];
        auto [x2, y2] = a[i + 1];
        if (i != k && x1 == x && y1 < y) {
            auto [x3, y3] = a[i - 1];
            if (x1 == x2 || (std::min(x1, x2) == std::min(x1, x3) && std::min(x1, x2) == x1) || (std::max(x1, x2) == std::max(x1, x3) && std::max(x1, x2) == x1))
                ;
            else
                cnt++;
        }
        if (x1 < x && x < x2) {
            if (y1 == y2 && y1 < y) {
                cnt++;
                continue;
            }
            if (x1 - x2 > 0) {
                if ((y - y2) * (x1 - x2) > (x - x2) * (y1 - y2))
                    cnt++;
            } else {
                if ((y - y2) * (x1 - x2) < (x - x2) * (y1 - y2))
                    cnt++;
            }
        }
    }
    // std::cerr << cnt << '\n';
    return cnt & 1 ^ 1;
}
void solve() {
    std::cin >> n;
    std::pair<ll, ll> a[n + 5];
    for (int i = 1; i <= n; i++) {
        std::cin >> a[i].first >> a[i].second;
    }
    a[n + 1] = a[1];
    a[0] = a[n];
    int ans = 0;
    bool vis = 0;
    for (int i = 0; i <= n; i++) {
        if (a[i + 1].second > a[i].second) {
            if (vis) {
                // std::cerr << i << " " << a[i].first << " " << a[i].second << " \n";
                ans += ck(i, a);
            }
            vis = 0;
        }
        if (a[i + 1].second < a[i].second)
            vis = 1;
    }
    std::cout << ans << '\n';
}
int main() {
    std::ios::sync_with_stdio(0);
    std::cin.tie(0), std::cout.tie(0);
    int t = 1;
    // std::cin >> t;
    while (t--) {
        solve();
    }
    std::cout << std::flush;
}