d = gets.chomp.to_i
c1 = gets.chomp
c2 = gets.chomp
c = c1 + c2
max_day = 0
(0..(c.length-d)).each do |i|
	count_day = 0
	(c[0, i] + ("o" * d) + c[i + d..-1]).each_char do |day|
		if day == 'x'
			count_day = 0
		else
			count_day += 1
			max_day = [count_day, max_day].max
		end
	end
end
puts max_day