S = input().strip()
nA = 0
nB = 0
for a in S:
    if a=="A":
        nA += 1
    else:
        nB += 1
if nA == nB:
    ans = len(S)
elif nA==0 or nB==0:
    ans = 0
else:
    C = [0]*len(S)
    if nA>nB:
        for i in range(len(S)):
            if S[i]=="B":
                C[i] = 1
        n0 = nA
        n1 = nB
    else:
        for i in range(len(S)):
            if S[i]=="A":
                C[i] = 1
        n0 = nB
        n1 = nA
    D = [0]*(len(S)+1)
    for i in range(len(S)):
        if C[i]==1:
            D[i+1] = D[i]+1
        else:
            D[i+1] = D[i]
    low = 0
    high = n1+1
    while low+1<high:
        mid = (low+high)//2
        flag = False
        for i in range(len(S)-2*mid+1):
            x = D[i+2*mid]-D[i]
            if x==mid:
                flag = True
                break
        if flag:
            low = mid
        else:
            high = mid
    ans = 2*low
print(ans)