from collections import defaultdict

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

        diff.append((cnt,i+1))

    d = defaultdict(int)
    ans = 0

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

    print(ans)
    






if __name__ == "__main__":
    main()