from collections import defaultdict

inf=1<<30
mi=defaultdict(lambda:inf)
ma=defaultdict(lambda:-inf)
S=input()
h=0
N=len(S)
mi[h]=0
ma[h]=0
for i in range(N):
    h+=(S[i]=="A")*2-1
    mi[h]=min(mi[h],i+1)
    ma[h]=max(ma[h],i+1)
ans=0
for h in mi:
    ans=max(ans,ma[h]-mi[h])
print(ans)