from math import *

X, Y, Z, W = map(int, input().split())
eps = 10**-6

yes = eps
no = 10 ** 18
while no - yes > eps:
    mid = (yes + no)/2
    if sqrt(mid ** 2 + Y ** 2) < sqrt((X/mid)**2 + Z ** 2) + W:
        yes = mid
    else:
        no = mid
t = yes
a = sqrt(Y ** 2 + t ** 2)
b = sqrt(Z ** 2 + (X / t) ** 2)
c = sqrt((X/t - Y) ** 2 + (t - Z) ** 2)
s = (a + b + c)/2
print(sqrt(s * (s - a) * (s - b) * (s - c)))