from collections import defaultdict

S = input()
cnt = [0]
mark_dict = defaultdict(int)
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 cnt_i == 0:
        ans = max(ans, i+1)
    else:
        if mark_dict[cnt_i] == 0:
            mark_dict[cnt_i] = i
        else:
            ans = max(ans, i - mark_dict[cnt_i])

print(ans)