file = open('20.txt')
s = file.readline()
l = 1
k = 0
ans = 0
for r in range(1, len(s)):
    if s[r-1:r+1] == 'AB':
        k += 1
    while k >= 50:
        if s[l-1:l+1] == 'AB':
            k -= 1
        l += 1
    ans = max(ans, r - l + 1)
print(ans)