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


public class Main_yukicoder450 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int vl = sc.nextInt();
		int vr = sc.nextInt();
		int d = sc.nextInt();
		int w = sc.nextInt();

		double t = (double)d / (vl + vr);
		double ret = t * w;

//		pr.printf("%.7f\n", ret);
		pr.println(ret);
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);

		solve();

		pr.close();
		sc.close();
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}