#include <bits/stdc++.h>
using namespace std;
int n;
vector<char> ans;
int b[100005], book[100005];
char ch[100005];
int check(int k) {
  memset(book, 0, sizeof(book));
  int len = strlen(ch);
  int flag = 1;
  for (int i = 0; i < len; i++) {
    if (ch[i] <= k)
      b[i] = 0;
    else
      b[i] = b[i - 1] + 1;
    if (b[i] >= n) flag = 0;
  }
  return flag;
}
int main() {
  scanf("%d", &n);
  scanf(" %s", ch);
  int len = strlen(ch);
  for (int i = 'a'; i <= 'z'; i++) {
    if (check(i)) {
      int first = -1, last = -1;
      for (int j = 0; j < len; j++) {
        if (ch[j] < i) {
          book[j]++;
          first = -1;
          last = -1;
        } else {
          if (first == -1) first = j;
          if (ch[j] == i) last = j;
          if (first != -1 && j - first == n - 1) {
            book[last] = 1;
            j = last;
            first = -1;
            last = -1;
          }
        }
      }
      for (int j = 0; j < len; j++) {
        if (book[j] || ch[j] < i) ans.push_back(ch[j]);
      }
      break;
    }
  }
  sort(ans.begin(), ans.end());
  for (int i = 0; i < ans.size(); i++) printf("%c", ans[i]);
  return 0;
}
