S = input()
if len(S) < 2 or S.count('A') == 0 or S.count('B') == 0:
    print('0')
else:
    rep = []
    for s in S:
        if s == 'A':
            rep.append(1)
        else:
            rep.append(-1)
    end = False
    for cutLength in range(2,len(S)+1)[::-1]:
        for startIndex in range(0,len(S)-cutLength+1):
            if sum(rep[startIndex:startIndex+cutLength]) == 0:
                print(cutLength)
                end = True
                break
        if end:
            break