x,y,z,w = map(int,input().split())
low = 1
high = x
for i in range(100):
    a = (low+high)/2
    b = x/a
    if (a**2+y**2)**(0.5)-w>(b**2+z**2)**(0.5):
        high = (high+low)/2
    else:
        low = (high+low)/2
s = (high+low)/2
t = x/s
print(x-(s*y)/2-(t-y)*(s-z)/2-(t*z)/2)