public static class Base26Converter
{
    private const string CharList = "abcdefghijklmnopqrstuvwxyz";

    public static String Encode(long input)
    {
        if (input < 0) throw new ArgumentOutOfRangeException("input", input, "input cannot be negative");

        char[] clistarr = CharList.ToCharArray();
        var result = new Stack<char>();
        while (input != 0)
        {
            result.Push(clistarr[input % 26]);
            input /= 26;
        }
        return new string(result.ToArray());
    }

    public static Int64 Decode(string input)
    {
        var reversed = input.ToLower().Reverse();
        long result = 0;
        int pos = 0;
        foreach (char c in reversed)
        {
            result += CharList.IndexOf(c) * (long)Math.Pow(26, pos);
            pos++;
        }
        return result;
    }
}