A=list(input())

CNT=len(A)
S=[len(A)+1 for i in range(len(A)*2+5)]
E=[0 for i in range(len(A)*2+5)]
for i in range(len(A)):
    if A[i]=="A":
        CNT+=1
    else:
        CNT-=1
    if E[CNT]<i:
        E[CNT]=i
    if S[CNT]>i:
        S[CNT]=i
S[0]=0
ans=0
for i in range(len(S)):
    if ans<(E[i]-S[i]):
        ans=(E[i]-S[i])

print(ans)