using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        try {

        F1();

        } catch (Exception ex)

        {

            Console.WriteLine("Top-level:" + ex.ToString());

        }

    }

    static void F1()

    {

        F2();

        F3();

    }

    static void F2() { F4(); }

    static void F3() { F4(); }

    static void F4() {

        try {

            throw new Exception("boo");

        } catch (Exception ex)

        {

            Console.WriteLine("Lower-level:" + ex.ToString());

            throw;

        }

    }

}