thanks...

#include <stdio.h>
#include <math.h>

#define PI 3.1416

int main()
{
        double a, r, area;

        scanf("%lf%lf", &a, &r);
        a /= 2;

        if (a >= r) {
                printf("%.3f\n", PI * r * r);
                return 0;
        }
        if (r >= sqrt(2) * a) {
                printf("%.3f\n", 4 * a * a);
                return 0;
        }
        printf("%.3f\n", PI * r * r - 4 * (acos(a / r) * r * r - a *
sqrt(r * r - a * a)));
        return 0;
}