Shell program to create a simple calculator

!/bin/bash
echo " Enter a number "
read a
echo " Enter another number "
read b
echo " Enter your choice 1.ADD 2.SUB 3.MULTIPLY 4.DIV"
read ch
if [ $ch -eq 1 ]
        then c=`expr $a + $b`

elif [ $ch -eq 2 ]
        then c=`expr $a - $b`
elif [ $ch -eq 3 ]
        then c=`expr $a\*$b`
elif [ $ch -eq 4 ]
        then c=`expr $a\$b`
fi
echo " The result is $c "