KaspichanNumbers

using System;
using System.Collections.Generic;

class KaspichanNumbers
{
    static void Main()
    {
        ulong inputN = ulong.Parse(Console.ReadLine());
        string bigLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        string smallLetters = "abcdefghijklmnopqrstuvwxyz";
        List<string> numbers = new List<string>(255);
        int counter = 25;
        for (int i = 0; i < 26; i++) numbers.Add(bigLetters[i].ToString());
        for (int i = 0; i < 26; i++)
        {
            for (int k = 0; k < 26; k++)
            {
                if(counter == 256) break;
                numbers.Add(smallLetters[i].ToString() + bigLetters[k]);
                counter++;
            }
            if (counter == 256) break;
        }

        List<string> output = new List<string>();
        if (inputN == 0) output.Add("A");
        while (inputN != 0)
        {
            output.Add(numbers[(int)(inputN % 256)]);
            inputN /= 256;
        }
        output.Reverse();
        Console.WriteLine(String.Join("",output));
    }
}