from math import sqrt,log,acos
def F(z):
    A=sqrt(4*R*R*z*z-D*D*H*H)

    V1=-4*D*H*R*z*A
    V2=8*R**3*z**3*acos(D*H/(2*R*z))
    V3=D**3*H**3*(2*log(R)+log(2*R*z+A))

    return (V1+V2+V3)/(12*H*H*R)

R,H,D=map(float,input().split())

if D<=2*R:
    print(F(H)-F(D*H/2*R))
else:
    print(0)