Polygon area

#include <iostream>
#include <vector>

using namespace std;


long long Area(vector<long long> X, vector<long long> Y, long long n)
{
    long long a = 0;

    for (long long i=0; i<n; i++){
        a += (X[i] * Y[(i+1)%n]) - (X[(i+1)%n] * Y[i]);
    }
 
    return a;
}

int main(){

    long long n;
    cin >> n;

    vector<long long> X(n);
    vector<long long> Y(n);

    for(long long i=0; i<n; i++)cin >> X[i] >> Y[i];

    cout << Area(X,Y,n) << endl;


}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: WRONG ANSWER