Rule 122.3 · Counters
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. See rule 704.
Ask the judge Pro
The life counter has an AI judge that answers your play citing the exact rules. It is part of Gatewatch Pro; the rules, Oracle text and rulings stay free.