from collections import defaultdict

S = list(input())

n = len(S)
C = defaultdict(list)
cnt = 0
C[0].append(0)
for i in range(n):
    if S[i] == 'A':
        cnt += 1
    else:
        cnt -= 1
    C[cnt].append(i + 1)
ans = 0
for L in C.values():
    ans = max(ans, max(L) - min(L))
print(ans)