probably easier in bash:
number="$1"
if [[ "$number" =~ "^(2|4|6|8|10|12|14|16|18|20)$" ]]; then
echo even
elif [[ "$number" =~ "^(1|3|5|7|9|11|13|15|17|19)$" ]]; then
echo odd
else
echo Nan
fi
A bit limited, but you can scale it up
Scaled up:
If $1 had a trailing non-digit, or was empty, that would indeed be an odd situation!