from math import sqrt,log,acos
def F():
    z=(D*H)/(2*R)
    delta=(H-z)/100000
    S=0
    while z<H:
        t=max(-1,min(1,D*H/(2*R*z)))
        alpha=4*R*z*acos(t)
        beta=D*H*sqrt(max(4-(D*H/(R*z))**2,0))

        S+=z*(alpha-beta)
        z+=delta
    return R*S/(2*H*H)*delta

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