import java.util.*;
class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int d = sc.nextInt();
		boolean[] day = new boolean[42];
		char[] in = sc.next().toCharArray();
		for(int i = 0; i < 7; i++) {
			day[14 + i] = (in[i] == 'o')?true:false;
		}
		in = sc.next().toCharArray();
		for(int i = 0; i < 7; i++) {
			day[21 + i] = (in[i] == 'o')?true:false;
		}
		int max = 0;
		for(int i = 0; i < 42; i++) {
			boolean[] cp = Arrays.copyOf(day, day.length);
			for(int j = 0; j < d; j++) {
				if(i + j >= cp.length) break;
				if(cp[i + j]) break;
				cp[i + j] = true;
			}
			
			for(int j = 0; j < 42; j++) {
				if(!cp[j]) continue;
				for(int k = j; k < 42; k++) {
					if(!cp[k]) {
						max = Math.max(max, k - j);
						break;
					}
				}
			}
			
			
			
		}
		System.out.println(max);
		
		
	}
}