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

main(){
	double pi=3.1415926535;
	double r,h,d,k,t,v;
	scanf("%lf%lf%lf",&r,&h,&d);
	t=0;
	if(d>=1e-8){
		k=d/r/2;
		t=-2*k*sqrt(1-k*k)-asin(k)+k*k*k*acosh(1/k);
	}
	v=h*r*r/3*(pi/2+t);
	printf("%f",v*2);
}