using System;

using System.Collections.Generic;

public class Program

{

    private List<char> _currentLetters = new List<char>() { '@' };

    public void Main()

    {

        for(int i = 10000; i >= 0; i--)

        {

            Console.WriteLine(GetLetter());

        }           

    }

    public string GetLetter()

    {

        bool incremented = false;

​

        for (int i = _currentLetters.Count - 1; i >= 0; i--)

        {

            if (_currentLetters[i] == 'Z')

            {

                _currentLetters[i] = 'A';

                continue;

            }

​

            _currentLetters[i] = (char)(_currentLetters[i] + 1);

            incremented = true;

            break;

        }

​

        if (incremented == false)

        {

            _currentLetters.Add('A');

        }

        return string.Join("", _currentLetters);

    }

}