Read four integer numbers from the user and find sum, product and average of these four numbers using Linux shell script in Ubuntu

echo Enter 4 Number :-
read n1 n2 n3 n4
SUM=`expr $n1 + $n2 + $n3 + $n4 `
PRODUCT=`expr $n1 \* $n2 \* $n3 \* $n4` 
AVG=`expr $SUM \/ 4`
echo Sum is :- $SUM
echo Product is :- $PRODUCT
echo Average is :- $AVG