#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e5 + 10;
char str[MAXN], ansstr[MAXN];
int n, num[MAXN], vis[MAXN];
struct Node {
  int l, r;
  int minval;
  Node(int l = 0, int r = 0, int minval = 0) : l(l), r(r), minval(minval){};
} node[MAXN * 4];
void build(int st, int ed, int k) {
  node[k].l = st;
  node[k].r = ed;
  if (st == ed) {
    node[k].minval = num[st];
    return;
  }
  int mid = (st + ed) >> 1;
  build(st, mid, (k << 1));
  build(mid + 1, ed, (k << 1 | 1));
  node[k].minval = min(node[(k << 1)].minval, node[(k << 1 | 1)].minval);
}
int query(int st, int ed, int k) {
  if (st == node[k].l && ed == node[k].r) {
    return node[k].minval;
  }
  if (node[(k << 1)].r >= ed) {
    return query(st, ed, (k << 1));
  } else if (node[(k << 1 | 1)].l <= st) {
    return query(st, ed, (k << 1 | 1));
  } else {
    return min(query(st, node[(k << 1)].r, (k << 1)),
               query(node[(k << 1 | 1)].l, ed, (k << 1 | 1)));
  }
}
int main() {
  while (~scanf("%d", &n)) {
    int val, pos, maxval = -1, minval = 0x3f3f3f3f;
    ;
    vector<int> ans;
    scanf("%s", str);
    int len = strlen(str);
    if (n > len) {
      for (int i = 0; i < len; i++) {
        minval = min(minval, (str[i] - 'a'));
      }
      ansstr[0] = (minval + 'a');
      ansstr[1] = '\0';
      printf("%s\n", ansstr);
      continue;
    }
    memset(vis, 0, sizeof(vis));
    for (int i = 0; i < len; i++) {
      num[i + 1] = str[i] - 'a';
    }
    build(1, len, 1);
    for (int i = 1; i <= len - n + 1; i++) {
      val = query(i, min(len, i + n - 1), 1);
      pos = 0;
      for (int j = i; j <= i + n - 1; j++) {
        if (num[j] == val) {
          pos = j;
        }
      }
      ans.push_back(pos);
      i = pos;
      vis[pos] = 1;
      maxval = max(maxval, num[pos]);
    }
    for (int i = 1; i <= len; i++) {
      if (num[i] < maxval && !vis[i]) {
        ans.push_back(i);
      }
    }
    for (int i = 0; i < ans.size(); i++) {
      ans[i] = num[ans[i]];
    }
    sort(ans.begin(), ans.end());
    for (int i = 0; i < ans.size(); i++) {
      ansstr[i] = (ans[i] + 'a');
    }
    ansstr[ans.size()] = '\0';
    printf("%s\n", ansstr);
  }
  return 0;
}
