using System;

public class Program

{

    public static void Main()

    {

        int input = 702;

        Console.WriteLine(Convert(input));

    }

    private static string Convert(int input){

        return Number2String((input-1)/26) + Number2String(input % 26 > 0 ? input % 26 : 26);

    }

    private static string Number2String(int number)

    {

        if(number > 0 && number <= 26){

            Char c = (Char)(65 + (number - 1));

            return c.ToString();

        }

        else

        {

            return "";

        }

    }

}

​