import java.util.*;

public class Main {
	public static void main (String[] args) {
    	Scanner sc = new Scanner(System.in);
    	double v1 = sc.nextInt();
    	double v2 = sc.nextInt();
    	double d = sc.nextInt();
    	double w = sc.nextInt();
    	System.out.println(d / (v1 + v2) * w);
	}
}