Gereva, Dimir Sewers

Gereva, Dimir Sewers enters the battlefiled tapped.
As you cast Gereva, Dimir Sewers you may pay {2}{u/b} if you do it gains Decoy.(when you cast this card, cast it into exie face-down with a decoy counter on it, if you do put a 2/2 colorless decoy creature token onto the battlefield it has "when this creature leaves the battlefield you may put a card you own with a decoy counter on it from exile onto the battlefield under your control")
{t}: add {u} or {b} to your mana pool.