import sys
from sys import stdin

X,Y,Z,W = map(int,stdin.readline().split())

l = Y
r = X+1

while r-l > 10**(-5):

    m = (l+r)/2

    apy = m
    bpz = X / m

    bp2 = Y**2 + bpz**2
    bq2 = Z**2 + apy**2

    #bp2が大きすぎる = bzが大きすぎる = ayが小さすぎる場合
    if bp2 > bq2 and bp2**0.5-bq2**0.5 > W:
        l = m
    else:
        r = m

a = (l+r)/2 - Y
b = X / (a+Y) - Z

print ( X - (a*b + (a+Y)*Z + Y*(b+Z) )/2 )