The sum of all fears

Hi I have several fields and I would like to know if there is
someway to
get the values of already typed text_fields and get it's sum, doing an
observer field for each text_field is gonna be kinda complicated.
If there is some way to use javascript for each time I type tab
uploading the value would be also desired
ps: the subject is only a way to get attetion =x

Please please don't do this. It's also a quick way to get people to
ignore you.