Problem Statement

There are N squares: square $1, square 2, \ldots, square N.$ Each square is colored red or blue.
The colors of the squares are represented by a string S of length N. Specifically, for each $i = 1, 2, \ldots, N, square i$ is colored red if the i-th character of S is R, and blue if it is B.
Square N is always colored blue.
Initially, each square has a chip of the same color as the square.
For each $i = 1, 2, \ldots, N-1$ in this order, let us perform the following operation.

• Choose an integer j such that $i \lt j \leq N$ and square j is colored blue, and place the pile of chips on square i on top of the pile of chips on square j, keeping the order of the chips.

Print the number of possible sequences of colors of chips of the N-chip pile on square N from top to bottom after the above process, modulo 998244353.

Input

The input is given from Standard Input in the following format:
N
S

Output

Print the answer.

Constraints

• $2 \leq N \leq 300$
• N is an integer.
• S is a string of length N consisting of R and B.
• The N-th character of S is B.

Sample Input 1

4
RBRB

Sample Output 1

2

We use a string consisting of R and B, corresponding to red and blue, to represent the sequence of colors of the chips of a pile from top to bottom.
Particularly, the sequence of colors of the chips of a 0-chip pile is an empty string $\varepsilon$.
Also, the state where the sequences of colors of chips of the piles on square 1, 2, 3, 4 are S_1, S_2, S_3, S_4, respectively, is called as state (S_1, S_2, S_3, S_4).
The following process yields RRBB as the sequence of colors of chips of the pile on square 4.

• First, on each square, place a chip of the same color as the square, resulting in state ( R , B , R , B ).
• Place the pile of chips on square 1 on top of the pile of chips on square 2, resulting in state $(\varepsilon, RB , R , B )$.
• Place the pile of chips on square 2 on top of the pile of chips on square 4, resulting in state $(\varepsilon, \varepsilon, R , RBB )$.
• Place the pile of chips on square 3 on top of the pile of chips on square 4, resulting in state $(\varepsilon, \varepsilon, \varepsilon, RRBB )$.

The following process yields RBRB as the sequence of colors of chips of the pile on square 4.

• First, on each square, place a chip of the same color as the square, resulting in state ( R , B , R , B ).
• Place the pile of chips on square 1 on top of the pile of chips on square 4, resulting in state $(\varepsilon, B , R , RB )$.
• Place the pile of chips on square 2 on top of the pile of chips on square 4, resulting in state $(\varepsilon, \varepsilon, R , BRB )$.
• Place the pile of chips on square 3 on top of the pile of chips on square 4, resulting in state $(\varepsilon, \varepsilon, \varepsilon, RBRB )$.

The above two, RRBB and RBRB, are the only possible sequences of colors of chips of the pile on square 4 from top to bottom after the process in the problem.

Sample Input 2

20
RRBRRRBBRBBBBRBRBRBB

Sample Output 2

92378

见题面 Input 部分

见题面 Output 部分