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

typedef long long LL;

struct point{
	double x, y;
};

double cross(point a, point b){
//    cout << a.x * b.y - b.x * a.y << endl;
    return a.x * b.y - b.x * a.y;
}

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];
    }
    
    vector<point> b;
    int lx = a[lid].first;
    int ly = a[lid].second;
    for(int i = lid + 1; i <= n + lid; i++){
    	int x = a[i].first;
    	int y = a[i].second;
    	b.push_back({x - lx, y - ly});
    	lx = x; ly = y;
	}

    int ans = 0;
    bool ok = false;
    point down = {0, 0};
    for(int i = 0; i < b.size(); i++){
//    	cout << i << endl;
//		cout << down.x << " " << down.y << endl;
//		cout << b[i].x << " " << b[i].y << endl;
//		cout << ok << endl;
    	if(b[i].y > 0){
			if(down.x != 0 || down.y != 0){
    			if(ok) ans++;
    			else if(cross(down, b[i]) >= 0){
    				ans++;
				}
			}
			ok = false;
			down = {0, 0};
		}else if(b[i].y < 0){
			down = b[i];
		}else if(b[i].x > 0){
			ok = true;
		}
//		cout << ok << endl;
//		cout << ans << endl;
//		cout << "-------" << endl;
	}
    
//    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;
}