import java.util.*;

class Ideone{
    public static void main (String[] args) throws java.lang.Exception
    {
        Scanner koko = new Scanner(System.in);
        int d = koko.nextInt();
        String one = koko.next();
        String two = koko.next();
        String whole = "xxxxxxxxxxxxxx" + one + two + "xxxxxxxxxxxxxx";
        if(whole.equals("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")){
        	System.out.println(d);
        }else{
        	String[] on = whole.split("o+");
		    int[] onseq = new int[on.length];
		    int counton = 0;
		    int countoff = 0;
		    int onp =0;
		    int offp =0;
		    for(int i=0; i<on.length; i++){
		    	if(on[i].length()!=0){
		    		onseq[onp++]=on[i].length();
		    		counton++;
		    	}
		    }
		    String[] off = whole.split("x+");
		    int[] offseq = new int[off.length];
		    for(int i=0; i<off.length; i++){
		    	if(off[i].length()!=0){
		    		offseq[offp++]=off[i].length();
		    		countoff++;
		    	}
		    }
		    int max=0;
		    
		    	if(onseq[0]<=d){
		    		max=onseq[0]+offseq[0];
		    	}
			    if(countoff==1){
			    	if(on[1].length()<=d){
			    		max=Math.max(max, offseq[0]+onseq[1]);
			    	}else{
			    		max=Math.max(max, offseq[0]+d);
			    	}
			    }else{
			    	for(int i=1; i<counton-1; i++){
			    		if(onseq[i]<=d){
			    			max=Math.max(max, offseq[i-1]+onseq[i]+offseq[i]);
			    		}else{
			    			max=Math.max(max, offseq[i-1]+d);
			    			max=Math.max(max, offseq[i]+d);
			    		}
			    	}
			    	max=Math.max(max, offseq[countoff-1]+Math.min(d, onseq[counton-1]));
			    }
		    System.out.println(max);
        
    }
    }
}