S=input()
N=len(S)
mp=dict({})
cum=0
mp[cum]=0
ans=0
for i in range(N):
    if S[i]=='A':
        cum+=1
    else :
        cum-=1
    if cum in mp.keys():
        ans = max(ans,i+1-mp[cum])
    else :
        mp[cum]=i+1

print(ans)