from math import sqrt,log,acos
def F(z):
    K=max(0,4*R*R*z*z-D*D*H*H)
    A=Decimal(sqrt(K))

    V1=-4*D*H*R*z*A

    L=D*H/(2*R*z)
    L=min(Decimal(1),max(Decimal(-1),L))
    V2=8*R**3*z**3*Decimal(acos(L))
    V3=D**3*H**3*(2*Decimal(log(R))+Decimal(log(2*R*z+A)))

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

from decimal import Decimal
R,H,D=map(Decimal,input().split())
print(F(H)-F(D*H/(2*R)))