s = list(input())

def f(s):
  count = 0
  total = 0
  while True:
    count = 0
    for i in range(len(s)-1):
      if s[i] == 'B':
        if s[i+1] == 'A':
          s[i], s[i+1] = s[i+1], s[i]
          count += 1
    total += count
    if count == 0:
      return total

print(f(s))