#! /usr/bin/env bash

letters=(A B C D E F G H I J K L M N O P Q R S T U V W X Y Z)
number=$1
while (( $number )); do
  numofnext=`expr \( $number - 1 \) % 26`
  echo -n ${letters[$numofnext]}
  number=`expr \( \( $number - 1 \) - $numofnext \) / 26`
done | rev