Millie, Witch of Calamity

When Millie, Witch of Calamity enters the battlefield, flip two coins. On heads each player sacrifices a land, on tails Millie gains +X/+0,
flying and haste until end of turn, where X is the number of lands in all graveyards.
Retrace (You may cast this card from your
graveyard by discarding a land card in addition to
paying its other costs.)