package no450;

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double vl = sc.nextDouble();
		double vr = sc.nextDouble();
		double d = sc.nextDouble();
		double w = sc.nextDouble();
		System.out.println(String.format("%.7f", d / (vl + vr) * w));
	}

}