using System;

using System.Linq;

//using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Luhn("1234567891"));

    }

​

    public static bool Luhn(string digits)

    {

        //string pattern = @"^[0-9]{10}$";

        //Regex regex = new Regex(pattern);

        //if (!regex.IsMatch(digits)) return false;

        return digits.Length == 10 && digits.All(char.IsDigit) &&

            digits.Reverse().Select(c => c - 48)

                .Select((num, i) =>

                    i % 2 == 0 ? num : ((num *= 2) > 9 ? num - 9 : num)

                ).Sum() % 10 == 0;

    }

}