The president exercises a check over Congress through his power to veto bills, but Congress may override any veto (excluding the so-called "pocket veto") by a two-thirds majority in each house. When the two houses of Congress cannot agree on a date for adjournment, the president may settle the dispute. Either house or both houses may be called into emergency session by the president. The Vice President serves as president of the Senate, but he may only vote to break a tie.

Answer this question, if possible (if impossible, reply "unanswerable"): Which veto is the only one Congress is able to override with a two thirds majority?
unanswerable