shell script to print sum of digits of the given num.In case of negative num is given as input, display Not a positive num as o/p

Question

I'm getting the proper output, but only for positive inputs.

read n
g=$n
s=0
k=
if [ $n -gt 0 ] ; then echo "Not a positive number"
else
    while [ $n -gt 0 ]
       do
          k=$CC $n % 10 ))
          n=$CC $n / 10 ))
          s=$(C $s + $k ))
       done
echo "Sum of digit for given number is $s"
fi