using System;

using System.Text.RegularExpressions;

public class Program

{

    public static int Modulus(int divisor, int dividend)

    {

        if (dividend == 0)

        {

            throw new DivideByZeroException();

        }

        int whole = (int)Math.Floor((double)divisor / (double)dividend);

        return divisor - (dividend * whole);

    }

    public static void Main()

    {

        Console.WriteLine(MOD3736.GetCheckDigit("0794"));

    }

    public static class MOD3736

            {

                private const string _alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

​

                private static readonly Regex _removeInvalidCharactersRegex = new Regex($"(?>[^{_alphabet}]+)");

​

                public static char GetCheckDigit(string s)

                {

                    if (s is null)

                    {

                        throw new ArgumentNullException(nameof(s));

                    }

                    if (s.Length == 0)

                    {

                        throw new ArgumentException(nameof(s));

                    }

​

                    var withoutInvalidCharacters = _removeInvalidCharactersRegex.Replace(s, string.Empty);

                    Console.WriteLine($"Input pulito: >{withoutInvalidCharacters}<");

                    if (withoutInvalidCharacters.Length == 0)

                    {

                        throw new ArgumentException(nameof(s));

                    }

                    int modu = 36;

                    int moduPlus1 = modu + 1;

                    int pj = modu;

                    for (int j = 0; j < s.Length; j++)

                    {

                        int a = _alphabet.IndexOf(s[j]); //s.Length - (j + 1)

                        int sj = (pj % moduPlus1) + a;

                        int pj_plus_1 = CustomRem(sj, modu) * 2;

                        Console.WriteLine($"Pj = {pj}\t\ta = {a}\t\tSj = {sj}\t\tpj+1 = {pj_plus_1 / 2}\t\t*2 = {pj_plus_1}\t\tpj+1 = {pj_plus_1 % 11}");

                        pj = pj_plus_1 % 11;

                    }

                    Console.WriteLine($"Pn = {pj}");

                    var xx = Modulus(1 - pj, modu);

                    Console.WriteLine($"x = {xx}");

                    return _alphabet[xx];

                }

                private static int CustomRem(int n1, int n2)

                {

                    var res = n1 % n2;

                    return res == 0 ? n2 : res;

                }

                public static bool IsValid(string s)

                {

                    return true;

                }

    }

}