MOD = 998244353

def solve(S):
    pos = []
    cnt = 0
    for i, ch in enumerate(S):
        if ch == 'A':
            pos.append(i + 1 - cnt)
            cnt += 1

    n = len(pos)
    if n == 0:
        return 1

    maxB = pos[-1]
    dp = [0] * (maxB + 1)
    dp[0] = 1

    for b in pos:
        ndp = [0] * (maxB + 1)
        pref = 0
        for x in range(b):
            pref = (pref + dp[x]) % MOD
            ndp[x] = pref
        dp = ndp

    return sum(dp[:pos[-1]]) % MOD