# -*- coding: utf-8 -*-
"""
No.450 ベー君のシャトルラン
https://yukicoder.me/problems/no/450

"""
import sys
from sys import stdin
from decimal import *
input = stdin.readline


def solve(v1, v2, d, w):
    getcontext().prec = 15
    ans = Decimal(d) / (Decimal(v1) + Decimal(v2)) * Decimal(w)
    return ans


def main(args):
    v1, v2 = map(float, input().split())
    d = float(input())
    w = float(input())
    ans = solve(v1, v2, d, w)
    print(ans)


if __name__ == '__main__':
    main(sys.argv[1:])