Project Euler #28 : Number spiral diagonals

int main() {
    IOS;
    llong sum = 1;
    llong last = 1;
    for (int i = 1; i <= 500; i++) {
        llong iv = i * 2;
        for (int j = 1; j <= 4; j++) {
            sum += last + iv;
            last += iv;
        }
    }
    cout << sum;
}