// https://keisan.casio.jp/exec/system/14748552221122

main(){
	double r,h,d,k,v;
	scanf("%lf%lf%lf",&r,&h,&d);
	k=d/r/2;
	v=h*r*r/3*(3.141592653589793238/2-2*k*sqrt(1-k*k)-asin(k)+k*k*k*acosh(1/k));
	printf("%f",v*2);
}