Why I have WA?

#include <iostream>
#include <cmath>

using namespace std;

int main() {
    double const pi = 3.14159265358979323846;
    double a;
    double r;
    cin >> a >> r;
    long double s = pi * r * r;
    if (r <= a / 2) {
        cout << roundl(s * 1000) / 1000;
    } else if (r >= ((a / 2) * sqrt(2))) {
        cout << (int) (a * a) << endl;
    } else {
        double cosx;
        cosx = (a / 2) / r;
        double rad;
        rad = acos(cosx) * 2;
        double segm = (r * r / 2) * (rad - sin(rad));
        s = s - 4 * segm;
        cout << round(s * 1000) / 1000 << endl;
    }
    return 0;
}