S = input()

n = len(S)

M = [-1] * (2 * n + 1)
m = [n] * (2 * n + 1)

x = 0
m[n] = -1
for i in range(n):
    if S[i] == 'A':
        x += 1
    else:
        x -= 1
    M[x + n] = i
    if m[x + n] == n:
        m[x + n] = i

print(max(M[i] - m[i] for i in range(2 * n + 1)))