a = gets.chomp
max_length = 0

(0..a.length).each do |i|
  descend_str, ascend_str = a[0, i-1].to_s, a[i-1, a.length].to_s

  balanced_length = descend_str.length if descend_str.count('A') == descend_str.count('B')
  max_length = balanced_length if ((balanced_length || 0) >= max_length)

  balanced_length = ascend_str.length if ascend_str.count('A') == ascend_str.count('B')
  max_length = balanced_length if ((balanced_length || 0) >= max_length)
end

puts max_length