D = int(raw_input())
C = list(raw_input()) + list(raw_input())
count = 0
max_holiday = 0
for i in range(14):
	if C[i] == "o":
		count += 1
	else:
		if i + D >= 14:
			count += 14 - i
		else:
			count += D
			for j in range(i+D,14):
				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