import java.util.*;
public class gw{
	public static void main(String... args){
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		String str1 = scan.next();
		String str2 = scan.next();
		System.out.println(calc(str1+str2,n));
	}
	public static int calc(String str,int n){
		String[] sp_o = str.split("x",0);//休日
		String[] sp_x = str.split("o",0);
		for(int i = 0; i<sp_x.length;i++){
			if(sp_x[i].length() <= n){
				return sp_x[i].length()+sp_x[i+1].length()+n; 
			}
			if(sp_x[i-1].length() > n && i > 0){
				return sp_o[i].length()+n;
			}
		}

		return 0;
	}
}