#!/usr/bin/env python3
vl, vr = map(int, input().split())
d = int(input())
w = int(input())
acc = 0
while d > 1e-8:
    dt = d / (w + vr)
    acc += dt * w
    d -= dt * (vl + vr)
    dt = d / (vl + w)
    acc += dt * w
    d -= dt * (vl + vr)
print(acc)