S = input()

A = [0 for _ in range(len(S) + 1)]
B = [0 for _ in range(len(S) + 1)]

for i in range(len(S)):
    if S[i] == 'A':
        A[i + 1] = 1
    else:
        B[i + 1] = 1

for i in range(len(S)):
    A[i + 1] += A[i]
    B[i + 1] += B[i]

D = [A[i] - B[i] for i in range(len(S) + 1)]

P = [[] for _ in range(len(S) + 1)]

for i in range(len(S) + 1):
    P[D[i]].append(i)

for i in range(len(S) + 1):
    P[i].sort()

res = 0

for i in range(len(S) + 1):
    if P[i]:
        res = max(res, P[i][-1] - P[i][0])

print(res)