Pollytis, the Eternal

at the end of each turn, if this creature has
greater power than it did at that turn's upkeep,
put that many+1/+1 counters on it.
at the end of each turn, if this creature has less
power than it did at that turn's upkeep, put
that many -1/-1 counters on it.
Counters cannot be put on this creature the
turn it is cast.