S = input()
a = {0: -1}
c = 0
max_len = 0
for i, s in enumerate(S):
    if s == "A":
        c += 1
    else:
        c -= 1
    if c in a:
        max_len = max(max_len, i - a[c])
    else:
        a[c] = i

print(max_len)