from fractions import Fraction

vl, vr = map(int, input().split())
d = int(input())
w = int(input())

D = d
ans = Fraction(0)
for _ in range(100):
	t = Fraction(D, w + vr)
	dd = t * (w - vl)
	tt = Fraction(dd, w + vl)
	D -= (t + tt) * (vr + vl)
	ans += (t + tt) * w

print(float(ans))