S = input()

from collections import defaultdict
d = defaultdict(int)

now = 0
ans = 0
d[now] = -1
for i in range(len(S)):
    if S[i] == "A":
        now += 1
    else:
        now -= 1
    if now in d:
        if i - d[now] > ans:
            ans = i - d[now]
    else:
        d[now] = i
print(ans)