#include "bits/stdc++.h"

#define rep(i,n) for(int i = 0;i < n;i++)
#define REP(i,n,k) for(int i = n;i < k;i++)
#define P(p) cout << (p) << endl;
#define sP(p) cout << setprecision(15) << fixed << p << endl;
#define pi 3.1415926535
#define IINF 1e9
#define LINF 1e18
#define vi vector<int>
#define mp make_pair

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
int dx[] = { 0, 1, 0, -1 };
int dy[] = { -1, 0, 1, 0 };

unsigned long long sttoi(std::string str) {
	unsigned long long ret;
	std::stringstream ss; ss << str;
	ss >> ret;
	return ret;
}

ull gcd(ull a, ull b){
	if (b > a)swap(a, b);
	if (b == 0) return a;
	return gcd(b, a%b);
}

void solve() {
  int n;
  cin >> n;
  string s, t;
  cin >> s >> t;
  s = "xxxxxxxxxxxxxxxxxxx" + s + t + "xxxxxxxxxxxxxxxxxxxx";
  int maxi = -1;
  rep(i, s.length()-14){
    int streak = 0;
    bool can = true;
    for (int k = 0; k <= n; k++){
      rep(j, k){
        if (s[i + j] == 'o')can = false;
      }
      if (can){
        string tmp = s;
        rep(j, k){
          tmp[i + j] = 'o';
        }
        streak = 0;
        rep(j, tmp.length()){
          if (tmp[j] == 'o'){
            streak++;
          }
          else{
            maxi = max(maxi, streak);
            streak = 0;
          }
        }
      }
    }
  }
  P(maxi);
}

int main() {
	solve();
	return 0;
}