#include <algorithm>
#include <vector>
#include <cfloat>
#include <string>
#include <cmath>
#include <set>
#include <cstdlib>
#include <map>
#include <ctime>
#include <iomanip>
#include <functional>
#include <deque>
#include <iostream>
#include <cstring>
#include <queue>
#include <cstdio>
#include <stack>
#include <climits>
#include <sys/time.h>
#include <cctype>

using namespace std;

typedef long long ll;

int main() {
  int d;
  cin >> d;
  string s, t;
  cin >> s;
  cin >> t;
  s = "xxxxxxxxxxxxxxxxxxxxxxxx"+s+t+"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
  t = s;

  int ans = 0, cnt = 0;
  for (int p = 0; p < s.size()-d+1; p++) {
    bool ok = true;
    for (int i = 0; i < d; i++) {
      if (s[p+i] == 'o') {
	ok = false;
	break;
      }
      s[p+i] = 'o';
    }
    if (!ok) {
      s = t;
    }

    cnt = 0;
    for (int i = 0; i < s.size(); i++) {
      if (s[i] == 'o') {
	cnt++;
      }else {
	ans = max(ans, cnt);
	cnt = 0;
      }
    }
    if (cnt == 8) {
      std::cout << p << std::endl;
    }
    ans = max(ans, cnt);
    s = t;
  }

  std::cout << ans << std::endl;
}