__author__ = 'amylase'

def solve(d, s):
    """
    solution for yukicoder No.204

    >>> solve(2, 'oxxoxxoooooxxo')
    8

    >>> solve(5, 'ooxxxxxooooooo')
    14

    >>> solve(1, 'oxxxxxoxoxxxxo')
    3

    >>> solve(14, 'xxxxxxxxxxxxxx')
    14

    >>> solve(0, 'o' * 14)
    14

    :param d: maximum continuous PTO
    :param s: calendar string
    :return: longest continuous holiday
    """
    n = len(s)

    answer = 0
    for i in range(n):
        j = i
        while j < n and s[j] == 'o' :
            j += 1

        j = min(j + d, n)
        """
        used = 0
        while j < n and s[j] == 'x' and used < d:
            j += 1
            used += 1
        """
        
        while j < n and s[j] == 'o' :
            j += 1

        answer = max(j - i, answer)

    return answer


if __name__ == '__main__':
    d = int(input())
    s = input() + input()
    print(solve(d, s))