How can I create a running tally?

I need to create a pdf where multiple users can access and add numbers to a running tally. I would like to make the total field calculate this number. Is there any way to do this with just two fields?

I was thinking field A would add any number entered in field B. Field B would add any number entered to field A then erase field B so the next person could add a number.

If not text fields I was wondering if buttons could do it, but cant find any way to get it to work.