#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
int m, len;
char s[N];
int cnt[256];
int check(char c) {
  int ret = 0;
  int last = -1, lastc = -1;
  for (int i = 0; i < len; i++) {
    if (s[i] < c)
      last = i;
    else if (s[i] == c) {
      if (i - last > m) {
        if (lastc != -1) {
          ret++;
          last = max(lastc, last);
          lastc = -1;
        }
        if (i - last > m) return -1;
      }
      if (i - last == m) {
        last = i;
        ret++;
        lastc = -1;
      } else
        lastc = i;
    } else {
      if (i - last >= m) {
        if (lastc != -1) {
          ret++;
          last = max(lastc, last);
          lastc = -1;
        }
        if (i - last >= m) return -1;
      }
    }
  }
  assert(ret > 0);
  return ret;
}
int main() {
  scanf("%d", &m);
  scanf("%s", s);
  len = strlen(s);
  for (int i = 0; i < len; i++) cnt[s[i]]++;
  for (char c = 'a'; c <= 'z'; c++)
    if (cnt[c]) {
      int t = check(c);
      if (t != -1) {
        for (char cc = 'a'; cc < c; cc++) {
          for (int i = 0; i < cnt[cc]; i++) putchar(cc);
        }
        while (t--) putchar(c);
        return 0;
      }
    }
  return 0;
}
