#include <bits/stdc++.h>
using namespace std;
mt19937 rnd(time(0));
const long long inf = 0x3f3f3f3f3f3f3f3fLL;
const long long N = 3e5 + 10;
const long long MOD = 1e9 + 7;
long long m, n, c[30];
string s;
long long go(long long x) {
  long long pre = -1, cur = -1, res = 0;
  for (long long i = 0; i < n; i++) {
    if (s[i] < 'a' + x) pre = i, cur = i;
    if (s[i] == 'a' + x) cur = i;
    if (i - pre == m) {
      if (cur > pre)
        res++, pre = cur;
      else
        return -1;
    }
  }
  return res;
}
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> m >> s;
  n = (long long)s.size();
  for (long long i = 0; i < n; i++) c[s[i] - 'a']++;
  for (long long i = 0; i < 26; i++) {
    long long x = go(i);
    if (x != -1) {
      for (long long j = 0; j < i; j++)
        for (long long k = 0; k < c[j]; k++) cout << (char)('a' + j);
      for (long long j = 0; j < x; j++) cout << (char)('a' + i);
      return 0;
    }
  }
  return 0;
}
