S=input().rstrip()
N=len(S)

res=0
c=dict()
c[0]=-1
A,B=0,0
for i in range(N):
    s=S[i]
    if s=="A":
        A+=1
    else:
        B+=1
    if A-B in c:
        res=max(i-c[A-B],res)
    else:
        c[A-B]=i

print(res)