Polygon area

#include <iostream>
#include <vector>

using namespace std;


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

    int prev = n - 1;
    for (int i=0; i<n; i++){
        a += (X[prev] + X[i]) * (Y[prev] - Y[i]);
        prev = i;
    }
 
    return abs(a / 2.0);
}

int main(){

    int n;
    cin >> n;

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

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

    cout << 2*polygonArea(X,Y,n) << endl;


}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: ACCEPTED