Creating a calculator with condition

So have I got this :

#!/bin/bash

clear


echo "Enter the first number:"
read n1

echo "Choose an operation:"
echo "1. add"
echo "2. subtract"
echo "3. multiply"
echo "4. divide"
read opr

echo "Enter the second number:"
read n2

if [ $opr = "1" ]
   then
      echo $((n1+n2))
elif [ $opr = "2" ]
   then
      echo $((n1-n2))
elif [ $opr = "3" ]
   then
      echo $((n1*n2))
elif [ $opr = "4" ]
   then
       echo $((n1/n2))
fi
exit 0

My only question is how do I add to my code so that when I try to divide by zero it will show Cannot divide by zero or invalid? Thanks!

This has somewhat the hallmarks of a homework-like question (in which case, if it is, please post this in the appropriate homework forum), but the simplest way to avoid dividing by zero is to test if the value you are dividing by is zero to begin with.

edit: Thank you. New thread here.