#!/bin/ksh -p

lc="abcdefghijklmnopqrstuvwxyz"
uc="ABCDEFGHIJKLMNOPQRSTUVWXYZ"

if [[ $1 == "" ]] ; then
  set 13;
fi

x=$((${1}+1))
while (( x > 26 )) ; do
  x=$((x - 26))
done
while (( x <= 0 )) ; do
  x=$((x + 26))
done

lc2=$(echo $lc$lc | awk '{print substr($1, '$x', 26)}')
uc2=$(echo $uc$uc | awk '{print substr($1, '$x', 26)}')

tr "$lc$uc" "$lc2$uc2"