def main():
    import sys
    input = sys.stdin.readline

    S = input().rstrip('\n')
    dic = {0: -1}
    tmp = 0
    ans = 0
    for i, s in enumerate(S):
        if s == 'A':
            tmp += 1
        else:
            tmp -= 1
        if tmp not in dic:
            dic[tmp] = i
        else:
            ans = max(ans, i - dic[tmp])
    print(ans)


if __name__ == '__main__':
    main()