S = input()
a = S.count('A')
b = len(S) - a

acum = [0]
bcum = [0]
for c in S:
    if c == 'A':
        acum.append(acum[-1] + 1)
        bcum.append(bcum[-1])
    else:
        acum.append(acum[-1])
        bcum.append(bcum[-1] + 1)

def judge(n):
    n *= 2
    for i in range(len(S)+1-n):
        if acum[i+n] - acum[i] == bcum[i+n] - bcum[i]:
            return True
    return False

ok = 0
ng = min(a,b)+1
while ng-ok > 1:
    m = (ng+ok) // 2
    if judge(m):
        ok = m
    else:
        ng = m
print(ok*2)