import java.util.Scanner;

public class prog {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int t1, t2, k1, k2;
        t1 = scan.nextInt();
        k1 =scan.nextInt();
        t2 =scan.nextInt();
        k2 =scan.nextInt();
        System.out.println(1/((float)(k1/t1) + (float)(k2/t2)));
    }
}