Polygon area

#include <bits/stdc++.h>

using namespace std;

int main() {

    int n;
    cin >> n;

    vector<int> x(n+1), y(n+1);

    for (int i = 0; i < n; i++)
        cin >> x[i] >> y[i];
    
    x[n] = x[0];
    y[n] = y[0];

    long long output = 0;

    for (int i = 0; i < n; i++) {
        output += x[i] * y[i+1] - x[i+1] * y[i];
    }

    cout << output << endl;

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: WRONG ANSWER