import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;

public class Main {
	
	public static long MOD = 1000000007;
	
	public static double[] run(double d, double w, double vl, double vr){
		double running = 0;
		
		final double go_right_t = d / (w + vr);
		running += w * go_right_t;
		d -= go_right_t * (vl + vr);
		
		final double go_left_t = d / (w + vl);
		running += w * go_left_t;
		d -= go_left_t * (vl + vr);
		
		return new double[]{d, running};
	}
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final double vl = sc.nextDouble();
		final double vr = sc.nextDouble();
		
		final double d = sc.nextDouble();
		final double w = sc.nextDouble();
		
		double[] first_try = run(d, w, vl, vr);
		final double second_d = first_try[0];
		
		double[] second_try = run(second_d, w, vl, vr);
		
		final double first_run  = first_try[1];
		final double second_run = second_try[1];
		
		final double rate = second_run / first_run;
		final double limit = first_run / (1 - rate);
		//System.out.println(first_run + " " + second_run + " " + rate + " " + limit);
		
		System.out.printf("%.8f\n", limit);
	}
}