Corvinian, Magic Professor

Corvinian, Magic Professor power is equal to
the number of instant, sorcery and enchantment
cards in all graveyards.
At the beggining of your upkeep you may pay
{u}{r} and discard an enchantment card. If you
do, choose an instant or sorcery card from your
graveyard with converted mana-cost less of the
discarded card. You may play it until the end of
turn without playing it's mana-cost.