import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Scanner;

class Main {

	public static void main(String[] args) {
		new Main().run();
	}
	
	void run() {
		Scanner sc = new Scanner(System.in);
		int D=sc.nextInt();
		char[] cs=(sc.next()+sc.next()).toCharArray();
		int[] a=new int[cs.length];
		int p=0;
		for(int i=0;i<cs.length;++i) {
			int j=i;
			while(j+1<cs.length&&cs[i]==cs[j+1])++j;
			a[p++]=j-i+1;
			i=j;
		}
		int cur=cs[0]=='x'?0:1;//x:平日
		int ans=0;
		for(int i=0;i<p;++i) {
			if(i%2==cur) {
				ans=Math.max(ans, (a[i]>=D&&i>0?a[i-1]:0)+Math.min(a[i],D)+(i+1<a.length?a[i+1]:0));
			}else {
				ans=Math.max(ans, a[i]+(i+1<a.length?(Math.min(a[i+1],D)+(i+2<a.length?a[i+2]:0)):0));
			}
		}
		System.out.println(ans);
	}

	void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
}