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

from math import *

def solve():
    a=r-d/2
    k=1-a/r
    res=h*r**2/3*(pi/2-2*k*(1-k**2)**0.5-asin(k)+k**3*acosh(1/k))
    return res*2

r,h,d=map(float,input().split())
print(solve())