#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.setf(ios::fixed);
  cout.precision(20);
  int m;
  cin >> m;
  string s;
  cin >> s;
  int n = s.size();
  vector<int> cnt(26);
  vector<int> have(n);
  for (int c = 0; c < 26; ++c) {
    for (int i = 0; i < n; ++i) {
      if (s[i] - 'a' == c) {
        have[i] = 1;
        ++cnt[c];
      }
    }
    bool fl = 1;
    int lst = -1;
    for (int i = 0; i < n; ++i) {
      if (have[i]) {
        fl &= (i - lst - 1 < m);
        lst = i;
      }
    }
    fl &= (n - lst - 1 < m);
    if (!fl) {
      continue;
    } else {
      int lst = -1;
      int nxt = 0;
      for (int i = 0; i < n; ++i) {
        nxt = max(nxt, i + 1);
        while (nxt < n && !have[nxt]) ++nxt;
        if (have[i] && s[i] - 'a' == c && nxt - lst - 1 < m) {
          --cnt[c];
        } else if (have[i])
          lst = i;
      }
      for (int i = 0; i < 26; ++i) {
        for (int j = 0; j < cnt[i]; ++j) {
          cout << char('a' + i);
        }
      }
      return 0;
    }
  }
}
