vl, vr = [ int(v) for v in input().split(" ") ]
d = int(input())
w = int(input())

t1 = d / (vr + w)
t2 =  (d - (vl + vr) * t1) / (w + vl)

a = w * (t1 + t2)
b = 1 - ( (vl + vr) * (t1 + t2) ) / d
total = a / (1 - b)


print(total)