import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	 static int D;
	 public static void main(String[] args) {
	      BufferedReader stdReader =new BufferedReader(new InputStreamReader(System.in));
	      boolean[] N = new boolean[14];
	      try {
	    	  D = Integer.parseInt(stdReader.readLine());
	    	  String[] temp1 = stdReader.readLine().split("");
	    	  String[] temp2 = stdReader.readLine().split("");
	    	 
	    	  int ans = 0;
	    	  for(int i=0;i<7;i++)
	    		  if(temp1[i].equals("o")) N[i] = true;
	    	  for(int i=7;i<14;i++)
	    		  if(temp2[i-7].equals("o")) N[i] = true;	    	  
	    	  ans = calc(-1,N);	    	  
	    	  for(int i=0;i<14;i++){
	    		  int count = 0;
	    		  for(int j=0;j<D && i+j <14 ;j++){
	    			  if(!N[i+j]){
	    				  count++;
	    				  N[i+j] = true;
	    			  }else{
	    				  break;
	    			  }
	    		  }	    			    		  
	    		  ans = Math.max(ans, calc(i,N));
	    		  for(int j=0;j<count;j++){
	    			  N[i+j] = false;
	    		  }
	    	  }
	    	  System.out.println(ans);
	      } catch (IOException e) {
			e.printStackTrace();
	      }
	}
	
	public static int calc(int num,boolean[] N){
		int count = 0;
		int ans = 0;
		if(num==-1){
			count = D;
			for(int i=0;i<N.length;i++){
				if(N[i]){
					count++;	
				}else{
					ans = Math.max(ans, count);
					count=0;
				}
			}
			ans = Math.max(ans, count);
			count = D;
			for(int i=N.length-1;i>=0;i--){
				if(N[i]){
					count++;	
				}else{
					ans = Math.max(ans, count);
					count=0;
				}
			}
		}else{		
			for(int i=0;i<N.length;i++){
				if(N[i]){
					count++;	
				}else{
					ans = Math.max(ans, count);
					count=0;
				}
			}	
			ans = Math.max(ans, count);
		}
		return ans;
	}
}