#include <bits/stdc++.h>
using namespace std;
void _dbg(string) { cout << endl; }
template <class H, class... T>
void _dbg(string s, H h, T... t) {
  int l = s.find(',');
  cout << s.substr(0, l) << " = " << h << ", ";
  _dbg(s.substr(l + 1), t...);
}
template <class T, class U>
ostream& operator<<(ostream& o, const pair<T, U>& p) {
  o << "(" << p.first << "," << p.second << ")";
  return o;
}
template <class T>
ostream& operator<<(ostream& o, const vector<T>& v) {
  o << "[";
  for (T t : v) {
    o << t << ",";
  }
  o << "]";
  return o;
}
const int N = 100000;
int n, m;
bool used[N + 5];
char s[N + 5];
int hoge(char c) {
  int crnt = 0;
  int last = -1;
  int ret = 0;
  for (int i = (int)(0); i < (int)(n); i++) {
    crnt++;
    if (s[i] == c) last = i;
    if (used[i]) crnt = 0;
    if (crnt == m) {
      if (i - last >= m) return -1;
      crnt = i - last;
      used[last] = true;
      ret++;
    }
  }
  return ret;
}
int main() {
  cin >> m >> s;
  n = strlen(s);
  for (int i = (int)(0); i < (int)(26); i++) {
    char c = i + 'a';
    int r = hoge(c);
    if (r == -1) {
      r = 0;
      for (int j = (int)(0); j < (int)(n); j++)
        if (s[j] == c) {
          r++;
          used[j] = true;
        }
    } else {
      i = 100;
    }
    for (int j = (int)(0); j < (int)(r); j++) printf("%c", c);
  }
  cout << endl;
  return 0;
}
