import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int vl = scan.nextInt();
		int vr = scan.nextInt();
		int d = scan.nextInt();
		int w = scan.nextInt();
		scan.close();
		double t = (double)d / (vl + vr) ;
		double ans = t * w;
		System.out.println(ans);
	}
}