import java.util.*;
class B{
	static Scanner s = new Scanner(System.in);
	public static void main(String[] args) {
		int
			vl=s.nextInt(),
			vr=s.nextInt(),
			d =s.nextInt(),
			w =s.nextInt();
		System.out.printf("%.10f\n",d*w*1.0/(vl+vr));
	}
}