If you join the imam as a latecomer and the imam stands for a fifth, do you follow him?

The questioner : If you join the imam and you are in the middle of the prayer, and the imam stands for the fifth, look, I am sure the imam stood for the fifth, and I am usually in a rak'ah that I am following in, and I do not intend the obligatory parts.

The Shaykh : No, no, follow him.

The questioner : Look, the imam's right is that you are in the fifth.

The Shaykh : We said that perhaps the imam did not add. Perhaps the imam left out the recitation of al-Fatihah in the first rak'ah. If he left out the recitation of al-Fatihah in the first rak'ah, the first rak'ah is invalid and the second becomes the first, so you and he are equal in the second. The second is more likely for him, and you may also follow.