#include <bits/stdc++.h>
using namespace std;
const long long infl = 4e18;
const long long infi = 1e9;
const long long mod = 1000000007;
auto clk = clock();
mt19937_64 rang(
    chrono::high_resolution_clock::now().time_since_epoch().count());
long long rng(long long lim) {
  uniform_int_distribution<long long> uid(0, lim - 1);
  return uid(rang);
}
long long powm(long long a, long long b) {
  long long res = 1;
  a %= mod;
  while (b) {
    if (b & 1) res = (res * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return res;
}
void solve() {
  long long n, m;
  string s;
  cin >> m >> s;
  n = ((long long)s.size());
  long long lo = 'a', hi = 'z', mid = (lo + hi) / 2;
  while (lo < hi) {
    bool valid = 1;
    long long last = -1;
    for (long long i = 0; i < n; i++) {
      if (s[i] <= mid) {
        last = i;
      }
      if (i - last >= m) valid = 0;
    }
    if (valid)
      hi = mid;
    else
      lo = mid + 1;
    mid = (lo + hi) / 2;
  }
  long long he = mid, last = -1, lastmota = -1;
  vector<char> ans;
  for (long long i = 0; i < n; i++) {
    if (s[i] < he) {
      last = i;
      ans.push_back(s[i]);
    } else if (s[i] == he) {
      lastmota = i;
    }
    if (i - last >= m) {
      last = lastmota;
      ans.push_back(he);
    }
  }
  sort(ans.begin(), ans.end());
  for (char i : ans) cout << i;
}
signed main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  srand(chrono::high_resolution_clock::now().time_since_epoch().count());
  cout << fixed << setprecision(10);
  long long t = 1;
  for (long long i = 1; i <= t; i++) solve();
  return 0;
}
