Sum of Series and Digits

#!/bin/bash
sum=0
echo -n "Sum of 1.Series 2.Digits: "
read n
case $n in
1):
    ip=1
    echo "Enter numbers to add. Press 0 to end."
    while [ $ip -ne 0 ]
    do
        read ip
        let sum+=ip
    done
    ;;
2):
    echo -n "Enter a number: "
    read ip
    while [ $ip -ne 0 ]
    do
        let r=ip%10
        let sum+=r
        let ip=ip/10
    done
    ;;
*):
    echo "Invalid choice"
    exit 1
    ;;
esac
echo "The sum is $sum"