Polygon area

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

double polygonArea(vector<double>& X, vector<double>& Y, int n)
{
    double area = 0.0;
 
    int j = n - 1;
    for (int i = 0; i < n; i++)
    {
        area += (X[j] + X[i]) * (Y[j] - Y[i]);
        j = i; 
    }
 
    return abs(area / 2.0);
}
 

int main()
{
    int n;
    cin >> n;

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

    for (int i = 0; i < n; i++)
    {
        cin >> X[i];
        cin >> Y[i];
    }
    
 
 
    cout << 2 * polygonArea(X, Y, n) << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: WRONG ANSWER