S = gets.strip

L = S.size

occurrences = Array.new( L * 2 + 2 ) { Array.new }
occurrences[L] << 0

depth = 0
L.times do |i|
	depth += if S[i] == 'A' then 1 else -1 end
	occurrences[ L + depth ] << i + 1
end

res = 0
occurrences.each do |ary|
	if !ary.empty? then
		res = [ res, ary.last - ary.first ].max
	end
end

puts res