f->Factorial F->Fibonacci S->Sum of Series D->Sum of Digits

#!/bin/bash
ans=0
echo -n "f->Factorial F->Fibonacci S->Sum of Series D->Sum of Digits: "
read n
case $n in
f):
        ans=1
        echo -n "Enter a number: "
        read ip
        while [ $ip -ne 1 ]
        do
                let ans*=ip
                let ip=ip-1
        done;;
F):
        a=0
        b=1
        echo -n "Enter the number of terms: "
        read ip
        if [ $ip -lt 1 ]
                then echo "invalid input"
                exit 1
        fi
        if [ $ip -ge 1 ]
                then echo -n "$a "
                let ans=a
        fi
        if [ $ip -ge 2 ]
                then echo -n " $b "
                let ans=b
        fi
        let ip=ip-2
        while [ $ip -gt 0 ]
        do
                let c=a+b
                echo -n " $c "
                a=b
                b=c
                let ans=c
                let ip=ip-1
        done
        ;;
S):
        ip=1
        echo "Enter numbers to add. Press 0 to end."
        while [ $ip -ne 0 ]
        do
                read ip
                let ans+=ip
        done;;
D):
        echo -n "Enter a number: "
        read ip
        if [ $ip -lt 0 ]
                then let ip=-ip
        fi
        while [ $ip -ne 0 ]
        do
                let r=ip%10
                let ans+=r
                let ip=ip/10
        done;;
*):
        echo "Invalid choice"
        exit 1;;
esac
echo "The answer is $ans."