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+1:
        E[CNT]=i+1
    if S[CNT]>i+1:
        S[CNT]=i+1
S[len(A)]=0
#print(S,E)
ans=0
for i in range(len(S)):
    if ans<(E[i]-S[i]):
        ans=(E[i]-S[i])

print(ans)