First strike, reach
At the beginning of combat on your turn, put X +1/+1 counters on another target creature you control, where X is Halana and Alena's power. That creature gains haste until end of turn.
If Halana and Alena's power is somehow negative at the time its triggered ability resolves, you will put no counters on that creature. The effect doesn't remove counters from that creature or give it -1/-1 counters.