S = input()
a = [None] * (len(S)+1)
a[0] = 0
c = 0
max_len = 0
for i, s in enumerate(S, 1):
    if s == "A":
        c += 1
    else:
        c -= 1
    if c in a:
        max_len = max(max_len, i - a.index(c))
    else:
        a[i] = c

print(max_len)