from itertools import accumulate

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

    d = list(accumulate(S, lambda s, c: s + 1 if c == 'A' else s-1))
    
    ans = 0
    for l in range(N-1):
        for r_plus1 in range(l+2, N+1):
            if d[r_plus1] - d[l] == 0:
                if r_plus1 - l > ans:
                    ans = r_plus1 - l
    print(ans)

main()