from math import sqrt,log,acos
def F():
    delta=0.0001
    z=(D*H)/(2*R)
    S=0
    while z<H:
        t=D*H/(2*R*z)
        alpha=4*R*z*acos(t)
        beta=D*H*sqrt(4-(D*H/(R*z))**2)
        S+=z*(alpha-beta)
        z+=delta
    return R*S/(2*H*H)*delta

from decimal import Decimal
R,H,D=map(float,input().split())
print(F())