Polygon area

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

int main() {
    int n;
    cin >> n;
    vector<pair<int, int>> vertices(n);
    for (int i = 0; i < n; ++i) {
        cin >> vertices[i].first >> vertices[i].second;
    }
    
    long long area = 0;
    for (int i = 0; i < n; ++i) {
        int j = (i + 1) % n;
        area += (long long)(vertices[i].first) * vertices[j].second;
        area -= (long long)(vertices[i].second) * vertices[j].first;
    }

    cout << abs(area) << endl;
    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED