S = input()
X = [1 << 19] * (1 << 19)
X[0] = 0
t = 0
ma = 0
for i, a in enumerate(S):
    if a == "A":
        t += 1
    else:
        t -= 1
    ma = max(ma, i - X[t])
    X[t] = min(X[t], i)
print(ma)