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