Polygon area

input/code.cpp: In function 'int main()':
input/code.cpp:24:19: warning: comparison of integer expressions of different signedness: 'int' and 'ull' {aka 'long long unsigned int'} [-Wsign-compare]
   24 |     for(int i=0; i<n-1; i++){
      |                  ~^~~~

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;

int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);

    ull n;
    cin >> n;

    ll sum = 0;

    vector<pair<ll,ll>> ps;
    ps.reserve(n);

    for(ull i=0; i<n; i++){
        ll x, y;
        cin >> x >> y;
        ps.push_back({x,y});
    }

    for(int i=0; i<n-1; i++){
        sum += ps[i].first * ps[i+1].second - ps[i+1].first * ps[i].second;
    }
    sum += ps[n-1].first * ps[0].second - ps[0].first * ps[n-1].second;
    cout << (abs(sum)) << "\n";
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED