import java.util.Arrays;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int d = sc.nextInt();
		char s1[] = sc.next().toCharArray();
		char s2[] = sc.next().toCharArray();
		char cal[] = new char[42];
		Arrays.fill(cal, 'x');
		for (int i = 14; i < 21; i++) {
			cal[i] = s1[i - 14];
		}
		for (int i = 21; i < 28; i++) {
			cal[i] = s2[i - 21];
		}
		char[] tmp;
		int cnt, max = 0;
		for (int i = 0; i <= 42 - d; i++) {
			tmp = Arrays.copyOf(cal, cal.length);
			cnt = 0;
			for (int j = i; j < d + i; j++) {
				if(tmp[j] == 'o') {
					break;
				}
				tmp[j] = 'o';
			}
			for (int j = 0; j < 42; j++) {
				if (tmp[j] == 'o') {
					cnt++;
					max = Math.max(max, cnt);
				} else {
					cnt = 0;
				}
			}
		}
		System.out.println(max);
	}
}