# -*- coding:utf-8 -*-

if __name__ == "__main__":
	ans = 0
	now = 0
	d = input()
	a = raw_input()
	b = raw_input()
	a = a + b
	now_o, now_x = 0,0
	prev_o,prev_x = 0,0
	mx = 0
	for i in a:
		if i == "o":
			if now_o == 0:
				prev_x = now_x
				now_x = 0
			now_o += 1
			if prev_x <= d:
				mx = max(mx,now_o + prev_o + prev_x)
			else:
				mx = max(mx,now_o+d)
		elif i == "x":
			if now_x == 0:
				prev_o = now_o
				now_o = 0
			now_x += 1
			if now_x <= d:
				mx = max(prev_o+now_x,mx)
	print mx