s=gets.chomp
n=s.size
a_num=s.count(?A)
b_num=n-a_num
if a_num==b_num
  puts n
  exit
end
if a_num==0||b_num==0
  puts 0
  exit
end

min_char,max_char,min_char_num=
    a_num<b_num ? [?A,?B,a_num] : [?B,?A,b_num]
min_char_pos=[*0...n].select{|i|s[i]==min_char}
until min_char_pos.empty?
  if min_char_pos.size==1
    puts 2
    exit
  end
  d=min_char_pos.last-min_char_pos.first
  if d>min_char_num*2
    d1=min_char_pos[1]-min_char_pos[0]
    d2=min_char_pos[-1]-min_char_pos[-2]
    if d1>d2
      min_char_pos.shift
    else
      min_char_pos.pop
    end
  else
    puts min_char_num*2
    exit
  end
end