S = input().strip()
C = [0]*(len(S)+1)
for i in range(len(S)):
    if S[i]=="A":
        C[i+1] = C[i]+1
    else:
        C[i+1] = C[i]-1
D = {}
for i in range(len(S)+1):
    if C[i] in D:
        D[C[i]].append(i)
    else:
        D[C[i]] = [i]
ans = 0
for k in D:
    ans = max(ans, D[k][-1]-D[k][0])
print(ans)