from collections import defaultdict

S = input()
mark_dict = defaultdict(list)
cnt = [0]
mark_dict[0].append(0)
ans = 0

for i in range(len(S)):
    if S[i] == 'A':
        cnt_i = cnt[-1] + 1
    else:
        cnt_i = cnt[-1] - 1
    
    cnt.append(cnt_i)
    
    if len(mark_dict[cnt_i]) == 0:
        mark_dict[cnt_i].append(i+1)
    else:
        ans = max(ans, i - mark_dict[cnt_i][0] + 1)

print(ans)