import java.util.*;

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