from collections import defaultdict
S = input()
ans = 0
a = 0
d = defaultdict(lambda: 0)
d[0] = -1
for i,c in enumerate(S):
    if c == 'A':
        a += 1
    else:
        a -= 1
    if a in d:
        ans = max(ans, i - d[a])
    else:
        d[a] = i
print(ans)