using System;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        var testString = "DFR25020014679444576824";

        var testSubstring = ParseRetailerCode(testString.Substring(8, 6));

        Console.WriteLine(testString);      

        Console.WriteLine(testSubstring);

    }

    public static string ParseRetailerCode(string tfvdRetailerCode)

    {

        if (tfvdRetailerCode.Substring(0, 1) == "0")

        {

            tfvdRetailerCode = tfvdRetailerCode.Substring(1, 4);

            return tfvdRetailerCode;

        }

        else

        {

            long storeIDValidate = Convert.ToInt64(tfvdRetailerCode.Substring(0, 5));

            int checkDigit = CalculateCheckDigit(storeIDValidate);

            if (checkDigit.ToString() == tfvdRetailerCode.Substring(5, 1))

                return tfvdRetailerCode;

            else

                return tfvdRetailerCode.Substring(0, 5);

        }

    }

    public static int CalculateCheckDigit(long number)

    {

        int checkDigit = 0;

        int cnt = 0;

        int rangeLength = 0;

        int total = 0;

        rangeLength = number.ToString().Length;

        int[] checkDigitArray = { 2, 3, 4, 5, 6, 7, 8, 9 };

        try

        {

            for (cnt = 0; cnt < rangeLength; cnt++)

            {

                total += int.Parse(number.ToString().Substring(rangeLength - 1 - cnt, 1)) * checkDigitArray[cnt];

            }

            checkDigit = total % 11;

            if (checkDigit == 10)

            {

                checkDigit = 0;

            }

            return checkDigit;

        }

        catch

        {

            //TODO: Implement proper catch statment

            throw;

        }

    }

}