using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string.Join(", ", GetColumns().Skip(1000).Take(60)).Dump();

    }

    public static IEnumerable<string> GetColumns()

    {

        int depth = 1;

        while (true)

        {

            foreach (var column in GetColumns(depth))

                yield return column;

            depth++;

        }

    }

    private static IEnumerable<string> GetColumns(int depth)

    {

        string letters = "abcdefghijklmnopqrstuvwxyz";

        foreach (var c in letters)

            if (depth == 1)         

                yield return c.ToString();

            else

            {

                foreach (var sub in GetColumns(depth - 1))

                    yield return c + sub;

            }

    }

}