S=input()
N=len(S)
D={0:[-1,-1]}

X=0
for i in range(N):
    if S[i]=="A":
        X+=1
    else:
        X-=1

    if X in D:
        D[X][1]=i
    else:
        D[X]=[i,i]

K=0
for x in D:
    K=max(K,D[x][1]-D[x][0])
print(K)