from collections import defaultdict

def main():
    S = input()
    cnt = 0
    ans = 0
    d = defaultdict(int)
    d[0] = -1
    for i, s in enumerate(S):
        if s == 'A':
            cnt += 1
        else:
            cnt -= 1

        if cnt in d:
            ans = max(ans, i - d[cnt])
        else:
            d[cnt] = i

    print(ans)
    






if __name__ == "__main__":
    main()