S = input()
ls = [0]
N = len(S)

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

ans = 0
lsa = [-1 for i in range(N+1)]

for i in range(N+1):
    if lsa[ls[i]] == -1:
        lsa[ls[i]] = i
    else:
        ans = max(ans,i-lsa[ls[i]])
print(ans)