S=input()
N=len(S)
X=[0]*(N+1)

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

D={}
for i in range(N+1):
    if X[i] in D:
        D[X[i]].append(i)
    else:
        D[X[i]]=[i]

M=0
for a in D:
    for i in range(len(D[a])-1):
        M=max(M,D[a][i+1]-D[a][i])
print(M)