Shared code

#include <bits/stdc++.h>
using namespace std;
#define st first
#define nd second
#define lb lower_bound
#define ub upper_bound
#define pb push_back
#define pf push_front
#define _pb pop_back
#define _pf pop_front
#define ll long long
#define ull unsigned long long
#define str string
#define db double
#define ldb long double
#define pi pair<int, int>
#define pll pair<ll, ll>
#define vi vector<int>
#define vll vector<ll>
#define vpi vector<pi>
#define vpll vector<pll>
#define gcd __gcd
#define lcm __lcm
#define file "TEST"
mt19937_64 rd(time(0));
template<typename T> T rand(T l, T r) { return uniform_int_distribution<T>(l, r)(rd); }
#define int long long
struct pt {
    int x, y;
};
bool cmp(pt a, pt b) {
    return a.x < b.x || (a.x == b.x && a.y < b.y);
}
bool cw(pt a, pt b, pt c) {
    return a.x*(b.y-c.y)+b.x*(c.y-a.y)+c.x*(a.y-b.y) < 0;
}
bool ccw(pt a, pt b, pt c) {
    return a.x*(b.y-c.y)+b.x*(c.y-a.y)+c.x*(a.y-b.y) > 0;
}
void convex_hull(vector<pt>& a) {
    if (a.size() == 1)
        return;

    sort(a.begin(), a.end(), &cmp);
    pt p1 = a[0], p2 = a.back();
    vector<pt> up, down;
    up.push_back(p1);
    down.push_back(p1);
    for (int i = 1; i < (int)a.size(); i++) {
        if (i == a.size() - 1 || cw(p1, a[i], p2)) {
            while (up.size() >= 2 && !cw(up[up.size()-2], up[up.size()-1], a[i]))
                up.pop_back();
            up.push_back(a[i]);
        }
        if (i == a.size() - 1 || ccw(p1, a[i], p2)) {
            while(down.size() >= 2 && !ccw(down[down.size()-2], down[down.size()-1], a[i]))
                down.pop_back();
            down.push_back(a[i]);
        }
    }

    a.clear();
    for (int i = 0; i < (int)up.size(); i++)
        a.push_back(up[i]);
    for (int i = down.size() - 2; i > 0; i--)
        a.push_back(down[i]);
}
int n;
vector<pt> a;
signed main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    // freopen(file".inp", "r", stdin);
    // freopen(file".out", "w", stdout);
    cin >> n;
    for (int i = 1; i <= n; ++i) {
        int X, Y;
        cin >> X >> Y;
        pt tmp;
        tmp.x = X, tmp.y = Y;
        a.pb(tmp);
    }
    convex_hull(a);
    cout << a.size() << '\n';
    for (auto i : a)
        cout << i.x << ' ' << i.y << '\n';
    return 0;
}