#!/usr/bin/env python3

import array
import copy
import itertools


MAX_D = 14
LEN_C = 14
NUM_DAYS = MAX_D + LEN_C + MAX_D


def compute_max_len_of_holidays(d, is_weekday, paid_start):
    assert paid_start < NUM_DAYS
    table = copy.copy(is_weekday)
    for idx in range(paid_start, NUM_DAYS):
        if not table[idx] or d == 0:
            break
        else:
            d -= 1
            table[idx] = False
    answer = 0
    for k, g in itertools.groupby(table):
        if not k:
            answer = max(answer, len(list(g)))
    return answer


def solve(d, is_weekday):
    answer = 0
    for idx in range(NUM_DAYS):
        answer = max(answer, compute_max_len_of_holidays(d, is_weekday, idx))
    return answer


def main():
    d = int(input())
    is_weekday = array.array("B", (True for _ in range(MAX_D)))
    is_weekday.extend(map(lambda c: c == "x", input()))
    is_weekday.extend(map(lambda c: c == "x", input()))
    is_weekday.extend(True for _ in range(MAX_D))
    print(solve(d, is_weekday))


if __name__ == '__main__':
    main()