Area of polygon

poly.cpp

double area(vector<int>& x, vector<int>& y) {
    int N = x.size();
    ll mulx = 0LL; ll muly = 0LL;
    for(int i = 0; i < N; i++) {
        mulx += (ll)x[i] * y[(i+1) % N];
    }
    for(int i = 0; i < N; i++) {
        muly += (ll)x[(i+1) % N] * y[i];
    }
    return 0.5 * llabs(mulx - muly);
}