def change(line)
	j = 0
	count = 0
	while true
		return count if j == line.length
		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{ count += change(line) }
puts count