import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

public class Main {
	
	public static final int SIZE = 14;
	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		final int D = sc.nextInt();
		
		boolean[] days = new boolean[SIZE];
		for(int i = 0; i < 2; i++){
			char[] inputs = sc.next().toCharArray();
			
			for(int j = 0; j < SIZE / 2; j++){
				days[i * (SIZE / 2) + j] = inputs[j] == 'o';
			}
		}
		
		int max = 0;
		for(int d = 0; d < SIZE - D; d++){
			boolean[] new_days = new boolean[SIZE];
			System.arraycopy(days, 0, new_days, 0, SIZE);
			
			for(int i = 0; i < D; i++){
				new_days[d + i] = true;
			}
			
			int prev = -1;
			for(int cur = 0; cur < SIZE; cur++){
				if(!new_days[cur] && prev >= 0){
					max = Math.max(max, cur - prev);
					prev = -1;
				}else if(new_days[cur] && prev < 0){
					prev = cur;
				}
			}
			
			if(prev >= 0){
				max = Math.max(max, SIZE - prev);
			}
		}
	
		System.out.println(max);
	}
}