from itertools import accumulate

def main():
    S = [0] + list(input())
    N = len(S)-1

    a = list(accumulate(S, lambda s, c: s + 1 if c == 'A' else s))
    b = list(accumulate(S, lambda s, c: s + 1 if c == 'B' else s))

    ans = 0
    for l in range(N-1):
        for r_plus1 in range(l+2, N+1):
            if a[r_plus1] - a[l] == b[r_plus1] - b[l]:
                d = r_plus1 - l
                if d > ans:
                    ans = d
    print(ans)

main()