from collections import defaultdict
s = input()
cnt = defaultdict(list)
cnt[0].append(0)

ans = 0
t = 0
for i in range(len(s)):
    if s[i] == 'A':
        t += 1
    else:
        t -= 1
    if len(cnt[t]) > 0:
        ans = max((i+1)-cnt[t][0],ans)
    cnt[t].append(i+1)

print(ans)