S=gets.chomp
def swapit(str)
  (str.length-1).times do |i|
    next if (str.length-1) == i
    if str[i] == 'B' && str[i+1] == 'A'
      str[i] = 'A'
      str[i+1] = 'B'
      return str
    end
  end
  return nil
end

t = 0
while true
  s1 = swapit(S)
  if s1
    t = t + 1
  else
    break
  end
end
puts t