Lexio Swap

You are given two strings $S$ and $T$, both of which contain only the letters '$A$' and '$B$'. You can perform the following operation on string $S$: - Choose an index $i$ $(1 \leq i \leq N-3$) such that $S_{i+1} = S_{i+2}$ and at the same time $S_i \neq S_{i+3}$. - Then, delete the characters at indices $i+1$ and $i+2$ from $S$. The two remaining parts are concatenated together. For example, if