Polygon area

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

int main() {
	int n;
	cin >> n;
	vector<ll> y;
	vector<ll> x;
	for (int i=0; i<n; i++) {
		ll a, b;
		cin >> a >> b;
		x.push_back(a);
		y.push_back(b);
	}
	x.push_back(x[0]);
	y.push_back(y[0]);
	ll a = 0;
	for (int i=0; i<n; i++) {
		a += x[i]*y[i+1] - x[i+1]*y[i];
	}
	cout << a;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: WRONG ANSWER