What should a latecomer recite in a loud or silent prayer if he does not know whether he is in the first or last rak'ahs?

The worshipper who misses rak'ahs reads Al-Fatihah and a surah in the rak'ahs he catches up with the Imam. Then he makes up for what he missed with Al-Fatihah only, because what he caught up with is the beginning of his prayer, and what he makes up for is its end, due to the saying of the Prophet, peace and blessings be upon him: "Whatever you catch up with, pray, and whatever you miss, complete it."