S=input()
N=len(S)
dp=[-1]*N*3
cum=0
dp[0]=0
ans=0
for i in range(N):
    if S[i]=='A':
        cum+=1
    else :
        cum-=1
    if dp[cum]!=-1:
        ans = max(ans,i+1-dp[cum])
    else :
        dp[cum]=i+1

print(ans)