from collections import *

S = input()
N = len(S)
A = [None] * N
for i in range(N):
    if S[i] == "A":
        A[i] = 1
    else:
        A[i] = -1
Ac = [0] * (N + 1)
for i in range(N):
    Ac[i + 1] = Ac[i] + A[i]
    
D = dict()
for i in range(N + 1):
    D[Ac[i]] = i
    
ans = 0
for i in range(N + 1):
    ans = max(ans, D[Ac[i]] - i)
print(ans)