using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(D3CustomerCheckDigit("052885069481400"));

        Console.WriteLine(D3CustomerCheckDigit("052709013207201"));

        Console.WriteLine(D3CustomerCheckDigit("056590673306002"));

        Console.WriteLine(D3CustomerCheckDigit("057545501500103"));

    }

    private static int D3CustomerCheckDigit(string idWithoutCheckdigit) {

    // allowable characters within identifier

    const string validChars = "0123456789ABCDEFGHIJKLMNOPQRSTUVYWXZ_";

    // remove leading or trailing whitespace, convert to uppercase

    idWithoutCheckdigit = idWithoutCheckdigit.Trim().ToUpper();

    // this will be a running total

    int sum = 0;

    // loop through digits from right to left

    for (int i = 0; i < idWithoutCheckdigit.Length; i++) {

        //set ch to "current" character to be processed

        char ch = idWithoutCheckdigit[idWithoutCheckdigit.Length - i - 1];

        // throw exception for invalid characters

        if (validChars.IndexOf(ch) == -1)

            throw new Exception(ch + " is an invalid character");

        // our "digit" is calculated using ASCII value - 48

        int digit = (int)ch - 48;

        // weight will be the current digit's contribution to

        // the running total

        int weight;

        if (i % 2 == 0)

        {

          // for alternating digits starting with the rightmost, we

          // use our formula this is the same as multiplying x 2 and

          // adding digits together for values 0 to 9.  Using the

          // following formula allows us to gracefully calculate a

          // weight for non-numeric "digits" as well (from their

          // ASCII value - 48).

          weight = (2 * digit) - (int) (digit / 5) * 9;

        }

        else

        {

            // even-positioned digits just contribute their ascii

            // value minus 48

            weight = digit;

        }

        // keep a running total of weights

        sum += weight;

    }

    // avoid sum less than 10 (if characters below "0" allowed,

    // this could happen)

    sum = Math.Abs(sum) + 10;

    // check digit is amount needed to reach next number

    // divisible by ten

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

}

}