s = input()
N = len(s)
ab = {0:-1}
a = 0
b = 0
ans = 0
for i in range(N):
    if s[i] == 'A':
        a += 1
    else:
        b += 1
    if a - b in ab:
        ans = max(ans,i - ab[a - b])
    else:
        ab[a - b] = i
print(ans)