package yukicoder;

import java.util.Scanner;

public class N450
{
	public static void main(String[] args)
	{
		Scanner sc =new Scanner (System.in);
		double v1=sc.nextInt(),v2=sc.nextInt();
		double d=sc.nextInt();
		double w=sc.nextInt();

		double ans=w*d/(v1+v2);

		System.out.printf("%.7f",ans);
	}

}