from collections import defaultdict

s = input()
l = [0] * (len(s) + 1)

for i in range(len(s)):
    if s[i] == 'A':
        l[i + 1] = l[i] + 1
    else:
        l[i + 1] = l[i] - 1

left = defaultdict(lambda: 10**10)
right = defaultdict(lambda: 0)

for i in range(len(s) + 1):
    left[l[i]] = min(left[l[i]], i)
    right[l[i]] = max(right[l[i]], i)

ans = 0
for k, v in left.items():
    ans = max(ans, right[k] - left[k])

print(ans)