Yet another String Problem

Every day some members of the ABA hacking cult enter the code $“ABA”$ for every successful hack that they perform. One fine day, some members messed it up by entering the sequence ABA at the same time which ultimately jinxed the file. The leader has to check if it file has been jinxed or not. The leader is given a string of length $N$ which is comprised only of A’s and B’s. He needs to figure out