import java.util.Scanner;

public class Main {
	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();
		char []c = new char[14];
		for(int i = 0; i < 7; i++) {
			c[i] = C1.charAt(i);
		}
		for(int i = 0; i < 7; i++) {
			c[i + 7] = C2.charAt(i);
		}
		int max = 0;
		for(int i = 0; i < 14; i++) {
			char []b = new char[14];
			for(int j = 0; j < 14; j++) {
				b[j] = c[j];
			}
			for(int j = i; j < i + D; j++) {
				if(j > 13) {
					break;
				}
				if(c[j] == 'x') {
					b[j] = 'o';
				}else {
					break;
				}
			}
			max = Math.max(max, solve(b));
		}
		int cnt1 = 0;
		for(int i = 0; i < 14; i++) {
			if(c[i] == 'o') {
				cnt1 ++;
			}else {
				break;
			}
		}
		int cnt2 = 0;
		for(int i = 13; i >= 0; i--) {
			if(c[i] == 'o') {
				cnt2++;
			}else {
				break;
			}
		}
		max = Math.max(max, cnt1 + D);
		max = Math.max(max, cnt2 + D);
		int cnt3 = 0;
		for(int i = 0; i < 14; i++) {
			if(c[i] == 'x') {
				cnt3 ++;
			}else {
				break;
			}
		}
		int cnt4 = 0;
		for(int i = 13; i >= 0; i--) {
			if(c[i] == 'x') {
				cnt4 ++;
			}else {
				break;
			}
		}
		if(cnt3 <= D) {
			int cnt5 = 0;
			for(int i = cnt3; i < 14; i++) {
				if(c[i] == 'o') {
					cnt5++;
				}else {
					break;
				}
			}
			max = Math.max(max, D + cnt5);
		}
		if(cnt4 <= D) {
			int cnt6 = 0;
			for(int i = 13 - cnt4; i >= 0; i--) {
				if(c[i] == 'o') {
					cnt6++;
				}else {
					break;
				}
			}
			max = Math.max(max, D + cnt6);
		}
		System.out.println(max);
	}
	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;
	}
}