#include <bits/stdc++.h>
using namespace std;
bool check(string& s, char c, int cnt, int m) {
  int last_seen_c = -1 - m;
  int cur_cov = -1;
  for (int i = 0; i < m - 1; ++i) {
    if (s[i] < c) {
      cur_cov = i + m - 1;
    } else if (s[i] == c) {
      last_seen_c = i;
    }
  }
  for (int i = m - 1; i < s.size(); ++i) {
    if (s[i] < c)
      cur_cov = i + m - 1;
    else if (s[i] == c) {
      last_seen_c = i;
      if (cur_cov < i) {
        if (cnt > 0)
          --cnt;
        else
          return false;
        cur_cov = i + m - 1;
      }
    } else if (cur_cov < i) {
      if (last_seen_c + m - 1 >= i && cnt > 0) {
        --cnt;
        cur_cov = last_seen_c + m - 1;
      } else
        return false;
    }
  }
  return true;
}
int main() {
  int m;
  cin >> m;
  string s;
  cin >> s;
  string ss = s;
  sort(ss.begin(), ss.end());
  std::vector<int> cnt(ss.size());
  cnt[0] = 1;
  for (int i = 1; i < cnt.size(); ++i) {
    if (ss[i] == ss[i - 1])
      cnt[i] = cnt[i - 1] + 1;
    else
      cnt[i] = 1;
  }
  int top = ss.size(), bot = 0;
  while (top > bot) {
    int mid = (top + bot) / 2;
    if (check(s, ss[mid], cnt[mid], m))
      top = mid;
    else
      bot = mid + 1;
  }
  cout << ss.substr(0, top + 1) << endl;
  return 0;
}
