S = gets.chomp

counter = Hash.new
counter[0] = -1

a_cnt = 0
b_cnt = 0
ans = 0

S.each_char.with_index do |s, i|
  if s == 'A'
    a_cnt += 1
  else
    b_cnt += 1
  end

  diff = a_cnt - b_cnt

  if counter[diff]
    len = i - counter[diff]
    ans = len if ans < len
  else
    counter[diff] = i
  end
end

puts ans