I need help in scoring

I need some help. I have this so far.

_root.score1 = 100
_root.score2 = 20

set as my background frame thing.
I have two text planes. One is score1 the other is score2.
I made a button that had this code.

on(release) {
score1 = score1 + 5
score2 = score2 - 5

All of that works perfectly, but then score2, once it reaches zero, it goes into negative.

How do I make it so that score2 doesn’t go into a negative number?
Is this clear enough?