using System;

namespace Yukicoder
{
    public class Program
    {
        public static void Main()
        {
            var v = Console.ReadLine().Split();
            double vL = int.Parse(v[0]);
            double vR = int.Parse(v[1]);
            double d = int.Parse(Console.ReadLine());
            double w = int.Parse(Console.ReadLine());
            double ans = w * (d / (vL + vR));
            Console.WriteLine(ans);
        }
    }
}