import sys
from itertools import accumulate

def input(): return sys.stdin.readline().strip()
def list2d(a, b, c): return [[c] * b for i in range(a)]
def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]
def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]
def ceil(x, y=1): return int(-(-x // y))
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]
def Yes(): print('Yes')
def No(): print('No')
def YES(): print('YES')
def NO(): print('NO')
sys.setrecursionlimit(10 ** 9)
INF = 10 ** 18
MOD = 10 ** 9 + 7
EPS = 10 ** -10

def bisearch_max(mn, mx, func):
    """ 条件を満たす最大値を見つける二分探索 """

    ok = mn
    ng = mx
    while ok+1 < ng:
        mid = (ok+ng) // 2
        if func(mid):
            ok = mid
        else:
            ng = mid
    return ok

S = input()
N = len(S)

acc = [0] * N
for i in range(N):
    if S[i] == 'A':
        acc[i] = 1
acc = [0] + list(accumulate(acc))

def check(m):
    m *= 2
    for i in range(N-m+1):
        cnta = acc[i+m] - acc[i]
        cntb = m - cnta
        if cnta == cntb:
            return True
    return False

res = bisearch_max(0, N//2+1, check)
ans = res * 2
print(ans)