using System;

using System.Text;

using System.Globalization;

using System.Numerics;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Base36Decode("US"));

        Console.WriteLine(Base36Encode(BigInteger.Parse("21911885219510514611218817625320142242185240")));

​

        for (int i = 0; i < (Clist.Length*4); i++)

        {

            Console.Write(i+"["+Base36Encode((ulong)i)+"] ");

            if ((i + 1) % 36 == 0) Console.WriteLine();

        }

​

    }

    private const string Clist = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    //private const string Clist = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

    private static readonly char[] Clistarr = Clist.ToCharArray();

    public static long Base36Decode(string inputString)

    {

        inputString = inputString.ToUpperInvariant();

        long result = 0;

        var pow = 0;

        for (var i = inputString.Length - 1; i >= 0; i--) {

            var c = inputString[i];

            var pos = Clist.IndexOf(c);

            if (pos > -1)

                result += pos * (long)Math.Pow(Clist.Length, pow);

            else

                return -1;

            pow++;

        }

        return result;

    }

    public static string Base36Encode(BigInteger inputNumber)

    {

        var sb = new StringBuilder();

        do {

            int i = (int)(inputNumber % (ulong)Clist.Length);

            sb.Append(Clistarr[i]);

            inputNumber /= (ulong)Clist.Length;

        } while (inputNumber != 0);

        return Reverse(sb.ToString());

    }

    public static string Reverse(string s)

    {

        var charArray = s.ToCharArray();

        Array.Reverse(charArray);

        return new string(charArray);

    }

}