s=input()
cnt=[0]*(len(s)+1)
now=0
for i in range(len(s)):
    if s[i]=='A':
        now+=1
    else:
        now-=1
    cnt[i+1]=now
if min(cnt)<0:
    d=min(cnt)
    for i in range(len(s)+1):
        cnt[i]+=d
ind=[-1]*(3*10**5)
ans=0
for i in range(len(s)+1):
    if ind[cnt[i]]==-1:
        ind[cnt[i]]=i
    else:
        ans=max(ans,i-ind[cnt[i]])
print(ans)