def longest_holidays(weeks):
    longest = 0
    x = 0
    for day in weeks:
        if day == 'o':
            x += 1
            if x > longest:
                longest = x
        else:
            x = 0
    return longest

d = int(input())
first_week = input()
second_week = input()
golden_weeks = first_week + second_week
longest = 0
for i in range(14 - d + 1):
    weeks = golden_weeks[:i] + 'o' * d + golden_weeks[i + d:]
    longest = max(longest_holidays(weeks), longest)
print(longest)