using System;

public class Program {
    public static void Main() {
        int[] a = IntArray();
        int[] b = IntArray();
        Console.WriteLine((a[0]*b[1]+a[1]*b[0])/(double)(a[0]+b[0]));
    }
    public static int[] IntArray() {
        return Array.ConvertAll(Console.ReadLine().Split(), delegate(string value) {
            return int.Parse(value);
        });
    }
}