#include <bits/stdc++.h>
using namespace std;
const int MAX = 1e5 + 10;
int n, m, cnt[26], kt, f[MAX];
deque<int> que;
bool fr[26];
char s[MAX];
bool check() {
  int pos = 0;
  for (int i = 1; i <= n + 1; ++i)
    if (i == n + 1 || !fr[s[i] - 'a']) {
      if (i - m > pos) return false;
      pos = i;
    }
  return true;
}
int main() {
  scanf("%d %s", &m, s + 1);
  n = strlen(s + 1);
  for (int i = 1; i <= n; ++i) ++cnt[s[i] - 'a'];
  for (int digit = 25; digit >= 0; --digit)
    if (cnt[digit] > 0) {
      fr[digit] = true;
      if (!check()) {
        fr[digit] = false;
        kt = digit;
        break;
      }
      cnt[digit] = 0;
    }
  int j = 1;
  que.push_back(0);
  for (int i = 1; i <= n + 1; ++i)
    if (i == n + 1 || !fr[s[i] - 'a']) {
      while (!que.empty() && i - m > que.front()) que.pop_front();
      f[i] = f[que.front()];
      if (i <= n && s[i] - 'a' == kt) ++f[i];
      while (!que.empty() && f[i] < f[que.back()]) que.pop_back();
      que.push_back(i);
    }
  cnt[kt] = f[n + 1];
  for (int digit = 0; digit < 26; ++digit)
    for (int sz = 0; sz < cnt[digit]; ++sz) printf("%c", 'a' + digit);
  return 0;
}
