import sys
from math import acos,sqrt
sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


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

M = H*(1-D/(2*R))
ans = 0
for i in range(1,10000001):
    h = M*i/10000000
    theta = acos(min(D/(2*R*(1-h/H)),1))
    ans += 2*theta*R**2*(1-h/H)**2-D*sqrt(abs((R*(1-h/H))**2-(D/2)**2))

print(M*ans/10000000)