S = input()
L = len(S)

A = [0]
for i in range(L):
    if S[i] == 'A':
        A.append(A[-1] + 1)
    else:
        A.append(A[-1] - 1)

ans = 0
d = dict()
for i in range(L+1):
    if A[i] not in d:
        d[A[i]] = i
    else:
        ans = max(ans, i - d[A[i]])
print(ans)