import java.util.*;

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