static string base10ToBase26(string s)
{
    char[] allowedLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();

    decimal num = Convert.ToDecimal(s);
    int reminder = 0;

    char[] result = new char[s.ToString().Length + 1];
    int j = 0;


    while ((num >= 26))
    {
        reminder = Convert.ToInt32(num % 26);
        result[j] = allowedLetters[reminder];
        num = (num - reminder) / 26;
        j += 1;
    }

    result[j] = allowedLetters[Convert.ToInt32(num)];

    string returnNum = "";

    for (int k = j; k >= 0; k -= 1) 
    {
        returnNum += result[k];
    }
    return returnNum;

}
static string base26ToBase10(string s)
{
    string allowedLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    System.Numerics.BigInteger result = new System.Numerics.BigInteger();

    for (int i = 0; i <= s.Length - 1; i += 1)
    {
        BigInteger pow = powof(26, (s.Length - i - 1));
        result = result + allowedLetters.IndexOf(s.Substring(i, 1)) * pow;
    }
    return result.ToString();
}
static BigInteger powof(int x, int y)
{
    BigInteger newNum = 1;

    if (y == 0)
    {
        return 1;
    }
    else if (y == 1)
    {
        return x;
    }
    else
    {
        for (int i = 0; i <= y - 1; i++)
        {
            newNum = newNum * x;
        }
        return newNum;
    }
}
static int modulo(long _num, long _base)
{
    return (int)(_num - _base * Convert.ToInt64(Math.Floor((decimal)_num / (decimal)_base)));
}