Liars Dice

At the beginning of your upkeep, exile the top card of your library facedown and look at it, then declare a number. Any opponent may call you a liar. If they do, reveal that card, and if that cards mana value is not the number declared, you take damage equal to that cards mana value and put it into your hand. If it is, that opponent takes damage equal to twice that cards mana value and you put it into your hand. If no one calls you a liar, you take damage equal to the declared number, then put that card i