Count The String

The alphabet of Zorpia is quite peculiar. It consists of only two letters, A and B and that too in upper-case only. A visitor to Zorpia is amazed by the diversity of words formed by the combination of A and B. He notices one more peculiar thing about the words. All the words were of even length and the number of A’s in the word is equal to the number of B’s. One of the natives told him that it is