# 二重ループはできない, 2*10**5
# 入力例1，AAABBA。+1 for A, -1 for Bとすれば
# levels = [0, 1, 2, 3, 2, 1, 2]
# 同じ数字で一番離れている区間が答え

S = input()
levels = [0]
for s in S:
    if s == 'A':
        levels.append(levels[-1]+1)
    if s == 'B':
        levels.append(levels[-1]-1)
        
#print(levels)

from collections import defaultdict
dic = defaultdict(list)
for i in range(len(levels)):
    dic[levels[i]].append(i)

#print(dic)

ans = 0
for d in dic:
    if len(dic[d]) >= 2:
        calc = dic[d][-1]-dic[d][0]
        ans = max(ans, calc)
print(ans)