S = input()

N = len(S)
L = [N]*(N+1)
ans = 0
memo = [-1]*(2*N+1)
memo[N] = 0
for i in range(N):
    if S[i] == "A":
        L[i+1] = L[i]+1
    else:
        L[i+1] = L[i]-1
    if memo[L[i+1]] < 0:
        memo[L[i+1]] = i+1
    else:
        ans = max(ans, i+1-memo[L[i+1]])
print(ans)