import java.util.*;

public class Exercise78{
  public static void main (String[] args){

    Scanner sc = new Scanner(System.in);

    double ax = sc.nextInt();
    double ay = sc.nextInt();
    double bx = sc.nextInt();
    double by = sc.nextInt();

    double height = ax / (ax + bx) * (by - ay);

    System.out.println(ay + height);
  }
}