#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 10;
char s[maxn], ans[maxn];
int n, c[maxn], pref[maxn];
void add(int x) {
  for (; x <= n; x += (x & (-x))) c[x]++;
}
int get(int x) {
  int tot = 0;
  for (; x; x -= (x & (-x))) tot += c[x];
  return tot;
}
char tree[maxn << 2];
void build(int p, int l, int r) {
  if (l >= r) {
    tree[p] = s[l - 1];
    return;
  }
  int m = (r + l) / 2;
  build((p << 1), l, m);
  build(((p << 1) | 1), m + 1, r);
  tree[p] = min(tree[(p << 1)], tree[((p << 1) | 1)]);
}
char query(int p, int l, int r, int s, int t) {
  if (s <= l && r <= t) return tree[p];
  int m = (r + l) / 2;
  char ans = 'z';
  if (s <= m) ans = min(ans, query((p << 1), l, m, s, t));
  if (t > m) ans = min(ans, query(((p << 1) | 1), m + 1, r, s, t));
  return ans;
}
int main() {
  int m;
  scanf("%d", &m);
  if (m < 1 || m > 100000) {
    printf("%d\n", 1 / 0);
    while (true)
      ;
    return 0;
  }
  scanf("%s", s);
  n = strlen(s);
  if (n < 0) {
    printf("%d\n", 1 / 0);
    while (true)
      ;
    return 0;
  }
  build(1, 1, n);
  char rp = 'a';
  for (int i = m; i <= n; i++) rp = max(rp, query(1, 1, n, i - m + 1, i));
  int tot = 0;
  int tmp = 0;
  for (int i = 0; s[i] != 0; i++) {
    if (s[i] < rp) {
      ans[tot++] = s[i];
      add(i + 1);
    }
    if (s[i] == rp) tmp = i;
    pref[i + 1] = tmp + 1;
  }
  int num = 0;
  for (int i = m; i <= n; i++) {
    if (get(i) - get(i - m) == 0) {
      num++;
      add(pref[i]);
    }
  }
  while (num--) ans[tot++] = rp;
  sort(ans, ans + tot);
  ans[tot] = 0;
  printf("%s\n", ans);
  return 0;
}
