#include <bits/stdc++.h>
using namespace std;
int n, m;
char s[100005];
vector<int> v[26];
int idx[26];
vector<int> w;
int main() {
  scanf("%d %s", &m, s);
  n = strlen(s);
  for (int i = 0; i < n; i++) v[s[i] - 'a'].push_back(i);
  memset(idx, 0, sizeof(idx));
  int p = 0;
  while (n - p >= m) {
    for (int i = 0; i < 26; i++) {
      while (idx[i] < v[i].size() && v[i][idx[i]] < p) idx[i]++;
      if (idx[i] == v[i].size()) continue;
      if (v[i][idx[i]] >= p + m) continue;
      while (idx[i] + 1 < v[i].size() && v[i][idx[i] + 1] < p + m) idx[i]++;
      p = v[i][idx[i]] + 1;
      w.push_back(i);
      break;
    }
  }
  int c = -1, cnt = 0;
  for (int x : w) {
    if (x > c) {
      c = x;
      cnt = 0;
    }
    if (x == c) cnt++;
  }
  vector<char> ans(cnt, 'a' + c);
  for (int i = 0; i < n; i++)
    if (s[i] - 'a' < c) ans.push_back(s[i]);
  sort(ans.begin(), ans.end());
  for (int i = 0; i < ans.size(); i++) s[i] = ans[i];
  s[ans.size()] = 0;
  puts(s);
}
