d = gets.chomp.to_i
c1 = gets.chomp
c2 = gets.chomp
c = c1 + c2
max_day = 0
def count_max_day(weeks)
	count_max_day = 0
	count_day = 0
	weeks.each_char do |day|
		if day == 'x'
			count_day = 0
		else
			count_day += 1
			count_max_day = [count_day, count_max_day].max
		end
	end
	count_max_day
end

max_day = 0
weeks = ('o' * d) + c
#puts weeks
max_day = [count_max_day(weeks), max_day].max
#p max_day
weeks = c + ('o' * d)
#puts weeks
#p max_day
max_day = [count_max_day(weeks), max_day].max
(0...(c.length-d)).each do |i|
	count_day = 0
	weeks = c[0, i] + ("o" * d) + c[i + d..-1]
	max_day = [count_max_day(weeks), max_day].max
end
puts max_day