/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] line = br.readLine().split(" ");
        float a = Integer.parseInt(line[0]);
        float b = Integer.parseInt(line[1]);
        line = br.readLine().split(" ");
        float c = Integer.parseInt(line[0]);
        float d = Integer.parseInt(line[1]);
        
        
        System.out.println((b*c+a*d)/(a+c));
    }
}