# -*- coding: utf-8 -*-

v1, v2 = map(int, raw_input().split())

d = int(raw_input())
w = int(raw_input())

assert 1 <= v1 <= 1e9
assert 1 <= v2 <= 1e9
assert 1 <= d <= 1e9
assert 1 <= w <= 1e9

print w * (float(d) / (v1 + v2))