import java.util.Scanner;

public class No450 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double v1 = sc.nextDouble();
		double v2 = sc.nextDouble();
		double d = sc.nextDouble();
		double w = sc.nextDouble();
		
		System.out.println(d * w / (v1 + v2));
	}
}