> #include <stdio.h>
> #include <math.h>
> void main(){
> double r,a,s,angl,cosa;
> scanf("%lf %lf",&a,&r);
> if (2*r<a) s=M_PI*r*r;
> else if (sqrt(2)*r>a) s=a*a;
> else{
>   cosa=a/(2*r);
>   angl=2*acos(cosa);
>   angl=M_PI/2-angl;
>   s=((a/2)*sqrt(r*r-(a*a)/4))+angl*r*r/2;
>   }
> printf("%.3lf\n",4*s);
> }

i think my reply was too late. but for other people.
when using sqrt(2) or something which arguments are not integer, you should use -> sqrt(2.0) instead.