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

print (maxCutLength)