Kicker

(You may pay an additional

as you cast this spell.)
Creatures you control get +1/+1 until end of turn. If this spell was kicked, Zombie creatures you control get an additional +2/+2 until end of turn.
To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.