def AB(S):
    oriS = S
    S = list(S)
    N = len(S)
    ans = 0
    i = 0
    while i <= N - 2:
        patS = S[i:i+2]
        if (patS == ['A', 'B']) or (patS == ['B', 'A']):
            S.pop(i)
            S.pop(i)
            N -= 2
        else:
            i += 1
    resS = ''
    for j in range(len(S)):
        resS += S[j]
    resN = len(resS)
    oriN = len(oriS)
    if (oriS[0] == resS[0]) or (oriS[-1] == resS[0]):
        if (oriS[:resN] == resS) or (oriS[oriN-resN:] == resS):
            ans = oriN - resN
    else:
        splS = resS[0] + resS + resS[0]
        splitted_S = oriS.split(splS)
        len1 = len(splitted_S[0])
        len2 = len(splitted_S[1])
        len_max = max([len1, len2])
        ans = len_max + 1
    return ans

def main():
    S = input()
    print(AB(S))

if __name__ == '__main__':
    main()