using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(GetLuhnCheckDigit("455673758689985"));

​

    }

    private static  string GetLuhnCheckDigit(string number)

        {

            var sum = 0;

            var alt = true;

            var digits = number.ToCharArray();

            for (int i = digits.Length - 1; i >= 0; i--)

            {

                var curDigit = (digits[i] - 48);

                if (alt)

                {

                    curDigit *= 2;

                    if (curDigit > 9)

                        curDigit -= 9;

                }

                sum += curDigit;

                alt = !alt;

            }

            if ((sum % 10) == 0 )

            {

                return "0";

            }

            return (10 - (sum % 10)).ToString();

        }

}