P17573 [JAG 2026 Summer Camp #3] ABbreviation

You are given a string $S$ consisting of `A` and `B`. You may perform the following operation on $S$ any number of times, possibly zero. - Choose an occurrence of `AB` as a contiguous substring of $S$, and replace these two characters with a single uppercase English letter (one of `A` through `Z`) of your choice. If the replacement letter is `A` or `B`, it may be used as part of `AB` in a later operation. For example, if $S$ is `AAB`, you may replace the last two characters `AB` with `C`, resulting in `AC`. If you replace them with `B` instead, the resulting string is `AB`, on which you may perform the operation again. Find the number of distinct strings that can be obtained from $S$ by performing the operation any number of times. Since the answer may be large, print it modulo $998244353$.

The input consists of a single test case of the following format. ```text N S ``` The first line contains an integer $N$ ($1\le N\le10^6$), representing the length of $S$. The second line contains a string $S$ of length $N$ consisting of `A` and `B`.

Print a single integer representing the number of distinct strings that can be obtained, modulo $998244353$.