test1=$(echo $num1 | sed 's/[0-9]//g')
test2=$(echo $num2 | sed 's/[0-9]//g')
if [ -n "$test1" -o -n "$test2" ];then
echo 'please input valid numbers'
exit
fi
if [ "$op" == '+' ];then
result=$(($num1+$num2))
elif [ "$op" == '-' ];then
result=$(($num1-$num2))
