using System;

using System.Text;

using System.Linq;                  

public class Program

{

 private static readonly char[] CharSet = "abcdefghijklmnopqrstuvwxyz".ToArray();

​

    private static string? IntToCombination(long combinationNum)

    {

        var result = new StringBuilder();

        int length = CharSet.Length;

        combinationNum--;

        if(combinationNum == 0) return "a";

        while (combinationNum > 0)

        {

            var remainder = combinationNum % length;

            result.Append(CharSet[remainder]);

            combinationNum = combinationNum / length;

        }

​

        return result.ToString();

    }

​

    static void Main()

    {

        for (long i = 1; i < 100; i++)

        {

            Console.WriteLine($"{i} = {IntToCombination(i)}");

        }

    }

}