#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std;

typedef pair<int,int> PII;
typedef vector<PII> VII;

PII operator-(PII a, PII b) {
    return {a.first-b.first, a.second-b.second};
}

int cross(PII a, PII b) {
    return a.first*b.second - a.second*b.first;
}

VII graham_scan(VII points) {
    int n=points.size();
    PII t=*min_element(points.begin(), points.end());

    vector<pair<double,PII>> angles;
    for (PII p : points) if (p!=t) {
        PII v = p-t;
        angles.push_back({atan2(v.second, v.first), p});
    }
    sort(angles.begin(), angles.end());

    vector<PII> hull = {t};
    for (auto [_, c] : angles) {
        while (hull.size()>=2) {
            PII a=hull[hull.size()-2], b=hull[hull.size()-1];
            PII ab=b-a, ac=c-a;
            if (cross(ab,ac)>0) break;
            hull.pop_back();
        }
        hull.push_back(c);
    }
    
    return hull;
}

int main() {
    vector<PII> points = {{4,0}, {2,3}, {5,2}, {6,1}, {8,4}, {6,6}, 
                          {5,4}, {4,5}, {2,6}, {1,1}, {1,5}, {3,2}};
    auto hull = graham_scan(points);
    for (auto [x,y] : hull) cout << x << ", " << y << endl;
    return 0;
}