S=input()
L = len(S)
d = {}
d[0] = 0
cnt = 0
ans = 0
for i in range(L):
    if S[i]=='A':
        cnt += 1
    else:
        cnt -=1
    if cnt not in d:
        # キーが存在しない場合の処理
        d[cnt] = i
    else:
        ans = max(ans,i-d[cnt])
print(ans)