#include <bits/stdc++.h>
using namespace std;
int II() {
  int q;
  scanf("%d", &q);
  return q;
}
long long LII() {
  long long q;
  scanf("%lld", &q);
  return q;
}
void solve() {
  int M;
  cin >> M;
  string s;
  cin >> s;
  int N = (int)s.size();
  deque<int> de[30];
  for (int i = 0; i < N; i++) {
    de[s[i] - 'a'].push_back(i);
  }
  map<int, int> mp;
  for (int i = 0; i < M; i++) mp[s[i] - 'a']++;
  string ss = "";
  int prev_mi = 100, prev_in;
  for (int i = 0; i < 26; i++) {
    if (mp[i] > 0) {
      prev_mi = i;
      ss += (char)(i + 'a');
      break;
    }
  }
  for (int i = M - 1; i >= 0; i--) {
    if ((s[i] - 'a') == prev_mi) {
      prev_in = i;
      break;
    }
  }
  de[s[0] - 'a'].pop_front();
  for (int i = 1; i < N; i++) {
    int q = i + M - 1;
    if (q >= N) break;
    mp[s[i - 1] - 'a']--;
    mp[s[i + M - 1] - 'a']++;
    int mi = 100;
    for (int j = 0; j < 26; j++) {
      if (mp[j] > 0) {
        mi = j;
        break;
      }
    }
    if (mi == prev_mi) {
      if (prev_in >= i) continue;
      int temp = 0;
      while (de[mi].size() > 0 && de[mi].front() <= q) {
        temp = de[mi].front();
        de[mi].pop_front();
      }
      prev_in = temp;
      ss += (char)(mi + 'a');
    } else {
      int temp = 0;
      while (de[mi].size() > 0 && de[mi].front() <= q) {
        temp = de[mi].front();
        de[mi].pop_front();
      }
      prev_in = temp;
      prev_mi = mi;
      ss += (mi + 'a');
    }
  }
  sort(ss.begin(), ss.end());
  char ch = ss.back();
  map<char, int> mp1;
  string s1 = "";
  for (auto it : s) mp1[it]++;
  for (char cc = 'a'; cc < ch; cc++) {
    int q = mp1[cc];
    while (q--) s1 += cc;
  }
  for (auto it : ss)
    if (it == ss.back()) s1 += it;
  cout << s1 << endl;
}
int32_t main() {
  int tt;
  tt = 1;
  int cas = 1;
  while (tt--) {
    solve();
  }
}
