def change(line)
	j = 0
	count = 0
	while true
		if j == line.length
			return count
		end

		if line[j] == "B" && line[j + 1] == "A"
			line[j] = "A"
			line[j + 1] = "B"
			count += 1
		end
		j += 1
	end	
end

line = gets.chomp.split("")

count = 0
line.length.times do
	count += change(line)
end
p count