#include <bits/stdc++.h>
using namespace std;
int x;
char s[100005];
bool flag[500];
int cnt[500];
vector<char> f;
int main() {
  scanf("%d", &x);
  scanf("%s", s);
  int len = strlen(s);
  for (int i = 0; i < len; i++) cnt[s[i]]++;
  for (int i = 'a'; i <= 'z'; i++) {
    if (cnt[i] == 0) continue;
    flag[i] = 1;
    int pre = -1;
    bool tag = 0;
    f.push_back(char(i));
    for (int j = 0; j < len; j++) {
      if (flag[s[j]]) {
        pre = j;
        continue;
      }
      int y = j - pre;
      if (y >= x) {
        tag = 1;
        break;
      }
    }
    if (tag)
      continue;
    else {
      cnt[i] = 0;
      flag[i] = 0;
      int pos = -1;
      int pre = -1;
      for (int j = 0; j < len; j++) {
        if (s[j] == i) {
          pos = max(pos, j);
        }
        if (flag[s[j]]) {
          pre = j;
          continue;
        }
        int y = j - pre;
        if (y >= x) {
          pre = pos;
          cnt[i]++;
        }
      }
      break;
    }
  }
  string ans = "";
  int p = f.size();
  for (int i = 0; i < p; i++) {
    for (int j = 1; j <= cnt[f[i]]; j++) ans += f[i];
  }
  cout << ans << endl;
  return 0;
}
