using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var rx = new Regex(@"\.txt$");

        Console.WriteLine(rx.IsMatch("abc"));

        Console.WriteLine(rx.IsMatch("abc.txt"));

        Console.WriteLine(rx.IsMatch("abc.what?"));

    }

}