Polygon area

#include <bits/stdc++.h>

using namespace std;

int main() {

    int n;
    cin >> n;

    vector<long long> x(n), y(n);

    for (int i = 0; i < n; i++) {
        int a, b;
        cin >> a >> b;
        x[i] = a;
        y[i] = b;
    }

    long long output = 0;

    output += (x[n-1] - x[0]) * (y[n-1] + y[0]);

    for (int i = 1; i < n; i++) {
        output += (x[i-1] - x[i]) * (y[i-1] + y[i]);
    }

    cout << output << endl;

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: WRONG ANSWER