import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class Main {
	static int []a;
	static int ans = 0;
	static List<Integer> list;
	static char[]c;
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int D = scan.nextInt();
		String C1 = scan.next();
		String C2 = scan.next();
		scan.close();
		c = new char[14];
		int cnt = 0;
		list = new ArrayList<Integer>();
		for(int i = 0; i < 7; i++) {
			char c1 = C1.charAt(i);
			char c2 = C2.charAt(i);
			c[i] = c1;
			c[i + 7] = c2;
			if(c1 == 'x') {
				cnt++;
				list.add(i);
			}
			if(c2 == 'x') {
				cnt++;
				list.add(i + 1);
			}
		}
		Collections.sort(list);
		if(cnt <= D) {
			System.out.println("14");
			System.exit(0);
		}
		a = new int[cnt];
		comb(1, D);
		System.out.println(ans);
	}
	static void comb(int m, int r) {
        int n = a.length;
        if(n == r) {
            return;
        }else if(n < r){
            System.out.println("エラー");
            System.exit(0);
        }
        if(m <= r) {
            int k = a[m - 1] + 1;
            for(int i = k; i <= n - r + m; i++){
                a[m] = i;
                comb(m + 1, r);
            }
        } else {
        	char[]b = new char[14];
        	for(int i = 0; i < 14; i++) {
        		b[i] = c[i];
        	}
        	for(int i = 1; i <= r; i++) {
        		//System.out.println(a[i]);
        		b[a[i] - 1] = 'o';
        	}
        	int t = solve(b);
        	if(ans < t) {
        		ans = t;
        	}
        }
    }
	static int solve(char []c) {
		int k = 0;
		int max = 0;
		for(int i = 0; i < 14; i++) {
			if(c[i] == 'o') {
				k++;
			}else {
				k = 0;
			}
			if(max < k) {
				max = k;
			}
		}
		return max;
	}
}