using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

​

        Predicate<string>f =

            s=>s[0]=='#'&s.Length>1&&(s[1]<48|s[1]>57)&s.Skip(1).All(x=>char.IsLetterOrDigit(x)|x=='_');

        Console.WriteLine(f("Hello World"));

        Console.WriteLine(f("#12thBday"));

        Console.WriteLine(f("#not-valid"));

        Console.WriteLine(f("#alsoNotValid!"));

        Console.WriteLine(f("#"));

        Console.WriteLine(f("#e"));

        Console.WriteLine(f("#_ABC"));

        Console.WriteLine(f("#thisisanunexpectedlylongstringxoxoxoxo"));

        Console.WriteLine(f("#USElection2016"));

    }

}