gistfile1.txt

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class Program
{
    private static ulong ConvertFromBaseToDecimal(string word, uint numeralSystem)
    {
        ulong power = 1;
        ulong decimalNumber = 0;
        for (int i = word.Length - 1; i >= 0; i--)
        {
            char letter = word[i];
            decimalNumber += ((ulong)(letter - 'a') * power);
            power *= numeralSystem;
        }

        return decimalNumber;
    }

    private static string ConvertFromDecimalToBase(ulong decimalNumber, uint numeralSystem)
    {
        StringBuilder baseNumberBuilder = new StringBuilder();
        while (decimalNumber > 0)
        {
            uint remainder = (uint)(decimalNumber % numeralSystem);
            char letter = (char)(remainder + 'a');
            baseNumberBuilder.Append(letter);
            decimalNumber /= numeralSystem;
        }

        return string.Join("", baseNumberBuilder.ToString().Reverse());
    }

    static void Main()
    {
        string[] wordsIn21Base = Console.ReadLine().Split(' ');

        List<string> result = new List<string>();
        for (int i = 0; i < wordsIn21Base.Length; i++)
        {
            ulong decimalNumber = ConvertFromBaseToDecimal(wordsIn21Base[i], 21);
            string numberAsHumanityWord = ConvertFromDecimalToBase(decimalNumber, 26);
            result.Add(numberAsHumanityWord);
        }

        Console.WriteLine(string.Join(" ", result));
    }
}