S = list(input().strip())

cnt = 0
while True:
  for i in range(len(S) - 1):
    if S[i] == 'B' and S[i+1] == 'A':
      S[i], S[i+1] = S[i+1], S[i]
      cnt += 1
      break
  else:
    break

print(cnt)