#include <bits/stdc++.h>
using namespace std;
vector<int> check(const vector<vector<int>> &mas, const vector<int> &answer,
                  const int &index, bool &flag, const string &s, const int &m) {
  vector<int> result = answer;
  for (int i = 0; i < mas[index].size(); i++) result.push_back(mas[index][i]);
  result.push_back(-1);
  result.push_back(s.length());
  sort(result.begin(), result.end());
  for (int i = 0; i < result.size() - 1; i++) {
    if (result[i] + m < result[i + 1]) {
      return result;
    }
  }
  flag = true;
  return answer;
}
int main() {
  int m;
  cin >> m;
  string s;
  cin >> s;
  vector<vector<int>> mas(26);
  vector<int> answer;
  vector<bool> used(s.length(), true);
  bool flag = false;
  for (int i = 0; i < s.length(); i++) mas[int(s[i]) - int('a')].push_back(i);
  for (int i = 0; i < 26; i++) {
    answer = check(mas, answer, i, flag, s, m);
    if (flag == true) {
      if (!answer.empty()) {
        for (int j = 0; j < s.length() - m + 1; j++) {
          auto it = lower_bound(answer.begin(), answer.end(), j + m);
          it--;
          if (*it >= j && *it <= j + m - 1)
            continue;
          else
            used[j] = false;
        }
      } else {
        used.clear();
        used.resize(s.length(), false);
      }
      for (int j = 0; j < s.length() - m + 1; j++) {
        if (used[j] == false) {
          auto it = lower_bound(mas[i].begin(), mas[i].end(), j + m);
          it--;
          answer.push_back(*it);
          j = *it;
        }
      }
      break;
    }
  }
  vector<char> ans;
  for (int i = 0; i < answer.size(); i++) {
    if (answer[i] != -1 && answer[i] != s.length()) ans.push_back(s[answer[i]]);
  }
  sort(ans.begin(), ans.end());
  for (int i = 0; i < ans.size(); i++) {
    cout << ans[i];
  }
  return 0;
}
