D = int(raw_input())
C = ["x" for i in range(14)] + list(raw_input()) + list(raw_input())
count = 0
max_holiday = 0
for i in range(28):
	if C[i] == "o":
		count += 1
	else:
		count += D
		for j in range(i+D,28):
			if C[j] == "o":
				count += 1
			else:
				break
		max_holiday = max(max_holiday, count)
		count = 0
else:
	max_holiday = max(max_holiday, count)
print max_holiday