operators usages

#! /bin/bash
if [ $# -ne 2 ];then
echo "please input two arguments:"
exit 2
fi
expr $1 + 2 &>/dev/null
if [ $? -ne 0 ];then
echo "please input int."
exit 3
fi
expr $2 + 3 &>/dev/null
if [ $? -ne 0 ];then
echo "please input int."
exit 4
fi

echo "$1+$2 is $(($1+$2))"
echo "$1$2 is $(($1$2))"
echo "$1-$2 is $(($1-$2))"
echo "$1/$2 is $(($1/$2))"