Kemi, Mischief Maker

At the beginning of your upkeep, you can choose any number. Flip a coin that
number of times. Each flip you win, Kemi
gets +1/+0 until your next upkeep. Each
flip you lose, Kemi gets -0/-1 until your next upkeep.
Artifact spells you cast cost {x} less to cast, where X is Kemi's power.