Janus, the Two-Faced God

You may pay X mana at the beginning of your first main phase. If you do, reveal the top two cards of your library. If either or both of the cards' converted mana costs are less than or equal to X, put that card on the battlefield. If not exile it. If white was paid for Janus, Janus has first strike. If blue was paid for Janus, Janus has flying.