using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Text;
using System.Text.RegularExpressions;
using System.Linq;

class Magatro
{
    static int v1, v2;
    static int d;
    static int w;
    static void Main()
    {
        string[] s = Console.ReadLine().Split(' ');
        v1 = int.Parse(s[0]);
        v2 = int.Parse(s[1]);
        d = int.Parse(Console.ReadLine());
        w = int.Parse(Console.ReadLine());
        Console.WriteLine((double)w * (double)d / (v1 + v2));
    }
}