I want to make sure I understand this rule:

122.3 If a permanent has both a 1/ 1 counter and a -1/-1 counter on it, N 1/ 1 and N -1/-1 counters are removed from it as a state-based action, where N is the smaller of the number of 1/ 1 and -1/-1 counters on it.

So to illustrate, suppose I have a Centaur Courser who has two -1/-1 counters on it. It gains a single 1/ 1 counter. The card now has three counters on it: 1 positive, two negative. One is smaller than two. Therefore I remove one -1/-1 counter and the 1/ 1 counter, leaving a single -1/-1 counter in place.

But, then I add one more 1/ 1 counter. The card now has two counters on it: one positive, one negative. One is not smaller than one. Therefore, both counters remain in play on the creature. Their net effect is zero, but both continue to exist.

It is only when I add one more 1/ 1 counter to the creature that there is an unequal number of counters on the card. 1 is less than 2, so the -1/-1 and a single 1/ 1 go away, leaving a single 1/ 1 in place.

Have I got that straight?

ETA: Okay, my question has been comprehensively answered, and I do not have that straight. They just cancel each other out. Thanks, all.