# -*- 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 = 0
	mx = 0
	for i in a:
		if i == "o":
			if now_o == 0:
				if now_x <= d:
					prev_o += now_x
				else:
					prev_o = d
				now_x = 0
			now_o += 1
		elif i == "x":
			if now_x == 0:
				mx = max(mx,now_o + prev_o)
				prev_o = now_o
				now_o = 0
			now_x += 1
	if now_x == 0:
		mx = max(mx,now_o + prev_o)
	print mx