import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int d = sc.nextInt();
		boolean[] days = new boolean[14];
		char[] first = sc.next().toCharArray();
		for (int i = 0; i < 7; i++) {
		    days[i] = (first[i] == 'o');
		}
		char[] second = sc.next().toCharArray();
		for (int i = 0; i < 7; i++) {
		    days[i + 7] = (second[i] == 'o');
		}
		int max = getLength(days);
		for (int i = 0; i < 14; i++) {
		    boolean[] tmp = (boolean[])(days.clone());
		    for (int j = 0; j < d && i + j < 14 && !tmp[i + j]; j++) {
		        tmp[i + j] = true;
		    }
		    max = Math.max(max, getLength(tmp));
		}
		System.out.println(max);
	}
	
	static int getLength(boolean[] days) {
	    int max = 0;
	    int length = 0;
	    for (boolean b : days) {
	        if (b) {
	            length++;
	            max = Math.max(max, length);
	        } else {
	            length = 0;
	        }
	    }
	    return max;
	}
}