Clockwork Swarm enters the battlefield with four +1/+0 counters on it. Clockwork Swarm can't be blocked by Walls. At end of combat, if Clockwork Swarm attacked or blocked this combat, remove a +1/+0 counter from it. , : Put up to X +1/+0 counters on Clockwork Swarm. This ability can't cause the total number of +1/+0 counters on Clockwork Swarm to be greater than four. Activate only during your upkeep.