a = gets.chomp
max_length = 0

(0..a.length).each do |i|
  descend_str_length = a[0, i].to_s.length
  descend_str_A = a[0, i].to_s.count('A')
  ascend_str_length = a[i, a.length].to_s.length
  ascend_str_A = a[i, a.length].to_s.count('A')

  balanced_length = descend_str_length / 2 == descend_str_A ? descend_str_A * 2 : 0
  max_length = balanced_length if balanced_length >= max_length

  balanced_length = ascend_str_length / 2 == ascend_str_A ? ascend_str_A * 2 : 0
  max_length = balanced_length if balanced_length >= max_length
end

puts max_length