#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstring>
#include <cmath>
#include <fstream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int v, w; };

ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

int f(string& s) {
	int maxi = 0;
	for (int i = 0; i <= 14; i++)
		for (int j = i; j <= 14; j++) {
			bool ok = true;
			for (int k = i; k < j; k++)
				if (s[k] == 'x')
					ok = false;
			if (ok) maxi = max(maxi, j - i);
		}
	return maxi;
}

int main() {
	int D; cin >> D;
	string s, t; cin >> s >> t;
	s = s + t;
	int maxi = 0;
	for (int i = 0; i + D <= 14; i++) {
		string _s = s;
		for (int j = i; j < i + D; j++)
			_s[j] = 'o';
		maxi = max(maxi, f(_s));
	}
	cout << maxi << endl;
}