import java.util.Scanner;

class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        double xa=sc.nextDouble(),ya=sc.nextDouble(),xb=sc.nextDouble(),yb=sc.nextDouble();
        sc.close();
        System.out.print((xb*ya+xa*yb)/(xa+xb));
    }
}