import java.io.PrintStream;
import java.util.Scanner;


public class Y450 {
	Scanner in = new Scanner(System.in);
	PrintStream out = new PrintStream(System.out);

	Y450() throws Exception {
		double vl = in.nextDouble();
		double vr = in.nextDouble();
		double d = in.nextDouble();
		double w = in.nextDouble();
		
		double t = d / (vl + vr);
		double dist = w * t;
		out.println(dist);
	}

	public static void main(String argv[]) throws Exception {
		new Y450();
	}
}