#include <bits/stdc++.h>
using namespace std;
double Fib(double x, double z);
string BinDec(long long x);
string StringInt(long long x);
unsigned long long StringInt(string x);
long long BinDec(string x);
unsigned long long POWMOD(unsigned long long x, unsigned long long y,
                          unsigned long long mod);
unsigned long long POWE(long long, long long);
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int x, i, j, q[30] = {}, qq[30] = {}, c[104000] = {}, s = 0;
  string a, ans;
  cin >> x >> a;
  for (i = 0; i < a.size(); i++) q[a[i] - 'a']++;
  for (i = 0; i <= a.size() - x; i++) {
    int d = 0, maxi = 1e6, k = 0;
    for (j = i; j < i + x; j++) {
      if (c[j] == 1) {
        d = 1;
        break;
      }
      if (a[j] <= maxi) {
        k = j;
        maxi = a[j];
      }
    }
    if (d == 0) {
      ans += a[k];
      c[k] = 1;
    }
  }
  bool r = 0;
  for (i = 0; i < ans.size(); i++) qq[ans[i] - 'a']++;
  for (i = 28; i >= 0; i--) {
    if (qq[i] != 0 && r == 0) {
      r = 1;
      continue;
    }
    if (r == 0) continue;
    for (j = 0; j < q[i] - qq[i]; j++) ans += (i + 'a');
  }
  sort(ans.begin(), ans.end());
  cout << ans;
  return 0;
}
double Fib(double x, double z) {
  int i = 0;
  double f = x, r = x, t;
  for (i = 2; i < z; i++) {
    t = f;
    f += r;
    r = t;
  }
  return f;
}
string BinDec(long long x) {
  string z;
  while (1) {
    if (x % 2 == 0)
      z += '0';
    else
      z += '1';
    x /= 2;
    if (x == 0) break;
  }
  reverse(z.begin(), z.end());
  return z;
}
long long BinDec(string x) {
  long long i;
  unsigned long long z = 1, a = 0;
  for (i = x.size() - 1; i >= 0; i--) {
    if (x[i] == '1') a += z;
    z *= 2;
  }
  return a;
}
unsigned long long StringInt(string x) {
  unsigned long long z = 0, i;
  for (i = 0; i < x.size(); i++) {
    z *= 10;
    z += (x[i] - '0');
  }
  return z;
}
string StringInt(long long x) {
  string z;
  while (x != 0) {
    z += (x % 10 + '0');
    x /= 10;
  }
  reverse(z.begin(), z.end());
  return z;
}
unsigned long long POWMOD(unsigned long long base, unsigned long long exp,
                          unsigned long long mod) {
  unsigned long long res = 1;
  while (exp > 0) {
    if (exp % 2 == 1) res = (res * base);
    res %= mod;
    exp /= 2;
    base = (base * base);
    base %= mod;
  }
  return res % mod;
}
unsigned long long POWE(long long base, long long exp) {
  unsigned long long res = 1;
  while (exp > 0) {
    if (exp % 2 == 1) res = (res * base);
    exp /= 2;
    base = (base * base);
  }
  return res;
}
