#include <bits/stdc++.h>
using namespace std;

typedef long long LL;

double cross(double x1, double y1, double x2, double y2){
    // cout << x1 << " " << y1 << endl;
    // cout << x2 << " " << y2 << endl;
    return x1 * y2 - x2 * y1;
}

void solve(){
    int n;
    cin >> n;
    int lid = 1;
    vector<pair<int, int>> a(2 * n + 1);
    for(int i = 1; i <= n; i++){
        cin >> a[i].first >> a[i].second;
        if(a[i].second > a[lid].second) lid = i;
        else if(a[i].second == a[lid].second && a[i].first < a[lid].first) lid = i;
    }
    for(int i = 1; i <= n; i++){
        a[n + i] = a[i];
    }

    int ans = 0;
    auto [lx, ly] = a[lid];
    for(int i = lid + 1; i <= n + lid; i++){
        auto [x, y] = a[i];
        auto [xx, yy] = a[i + 1];
        // cout << i << endl;
        // cout << lx << " " << ly << endl;
        // cout << x << " " << y << endl;
        // cout << xx << " " << yy << endl;
        if(y > ly){
            lx = x;
            ly = y;
        }
        if(yy > y && cross(x - lx, y - ly, xx - x, yy - y) >= 0){
            lx = xx;
            ly = yy;
            ans++;
        }
        // cout << ans << endl;
        // cout << "-------" << endl;
    }
    cout << ans << "\n";
    return;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int T = 1;
    while(T--){
        solve();
    }
    return 0;
}