Write a shell script of calculator which performs Addition , Subtract ,Division ,Multiplication using if..else statement - OS

echo " Enter one no."
read n1
echo "Enter second no."
read n2
echo "1.Addition"
echo "2.Subtraction"
echo "3.Multiplication"
echo "4.Division"
echo "Enter your choice"
read ch
if [ $ch = "1" ]
then
sum=`expr $n1 + $n2`
echo "Sum ="$sum
elif [ $ch = "2" ]
then
sum=`expr $n1 - $n2`
echo "Sub = "$sum
elif [ $ch = "3" ]
then
sum=`expr $n1 \* $n2`
echo "Mul = "$sum
elif [ $ch = "4" ]
then
sum=`expr $n1 / $n2`
echo "Div = "$sum
fi