Polygon area

#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define pb push_back
#define VI vector<int>
#define AR(x) array<int, x>
#define MX vector<VI >
#define all(x) x.begin(), x.end()
#define IOS cin.tie(0)->sync_with_stdio(0)

using namespace std;

using C = complex<ll>;

int main(){

    int n;
    cin >> n;

    vector<C> p(n);

    for(auto &c : p){
        ll x, y;
        cin >> x >> y;
        c = {x, y};
    }

    ll ans = 0;
    for(int i=0; i<n; i++){
        ans += p[i].real()*p[(i+1)%n].imag()-p[i].imag()*p[(i+1)%n].real();
    }

    cout << abs(ans) << '\n';


    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED