Polygon area

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

#define ll long long

int main()
{
    ll n;
    cin >> n;
    vector<pair<ll,ll>> v(n);
    for(ll i = 0; i < n; ++i){
        ll a,b;
        cin >> a >> b;
        v[i] = make_pair(a,b);
    }
    ll area = 0;

    for (ll i = 0; i < n; i++) {
        area += (v[i].first
                    * v[(i + 1) % n].second
                - v[(i + 1) % n].first * v[i].second);
    }
    if(area < 0) area = (-1)*area;
    cout << area<<endl;

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED