using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Text;
using System.Text.RegularExpressions;
using System.Linq;
using System.IO;

class Program
{
    static private Magatro M = new Magatro();
    static private void Main(string[]args)
    {
        M.Scan();
        M.Solve();
    }
}

public class Scanner
{
    private string[] S;
    private int Index;
    private char Separator;

    public Scanner(char separator = ' ')
    {
        Index = 0;
        Separator = separator;
    }

    private string[] Line()
    {
        return Console.ReadLine().Split(Separator);
    }

    public string Next()
    {
        string result;
        if (S == null || Index >= S.Length)
        {
            S = Line();
            Index = 0;
        }
        result = S[Index];
        Index++;
        return result;
    }
    public int NextInt()
    {
        return int.Parse(Next());
    }
    public double NextDouble()
    {
        return double.Parse(Next());
    }
    public long NextLong()
    {
        return long.Parse(Next());
    }
}

public class Magatro
{
    private int D;
    private bool[] C;
    public void Scan()
    {
        Scanner sc = new Scanner();
        D = sc.NextInt();
        C = new bool[14 + D * 2];
        for(int i = 0; i < 2; i++)
        {
            string line = sc.Next();
            for(int j = 0; j < 7; j++)
            {
                C[i * 7 + j + D] = line[j] == 'o';
            }

        }
    }

    public void Solve()
    {
        int ans = 0;
     for(int i = 0; i < C.Length - D; i++)
        {
            ans = Math.Max(ans, Count(i));
        }
        Console.WriteLine(ans);
    }
    private int Count(int index)
    {
        bool[] cp = C.ToArray();
        for(int i = index; i < index + D; i++)
        {
            if (cp[i])
            {
                return -1;
            }
            cp[i] = true;
        }
        int cnt = 0;
        int result = 0;
        for(int i = 0; i < cp.Length; i++)
        {
            if (cp[i]) cnt++;
            else
            {
                result = Math.Max(result, cnt);
                cnt = 0;
            }
        }
        result = Math.Max(result, cnt);
        return result;
    }
}