#include <bits/stdc++.h>
struct read_item {
  read_item(){};
  template <class T>
  operator T() {
    T t;
    std::cin >> t;
    return t;
  }
};
char splf(int i, int n) { return (i + 1 < n) ? ' ' : '\n'; };
struct RI {
  int i;
  RI(int a) : i(a) {}
  int operator*() { return i; }
  void operator++() { i += 1; }
  bool operator!=(const RI& r) { return i != r.i; }
};
struct range {
  RI x, y;
  RI begin() { return x; }
  RI end() { return y; }
  range(int a, int b) : x(a), y(b) {}
  range(int a) : x(0), y(a) {}
};
using namespace std;
int main() {
  int m = read_item();
  string s = read_item();
  int len = s.size();
  vector<bool> used(len, false);
  for (auto lim : range(26)) {
    fill(used.begin(), used.end(), false);
    bool ok = true;
    int prev = -1;
    int candidate = -1;
    for (auto i : range(len)) {
      int c = s[i] - 'a';
      if (c <= lim - 1) {
        prev = i;
        candidate = -1;
        used[i] = true;
        continue;
      }
      if (c == lim) {
        candidate = i;
      }
      if (prev + m == i) {
        if (candidate >= 0) {
          used[candidate] = true;
          prev = candidate;
          candidate = -1;
          continue;
        }
        ok = false;
        break;
      }
    }
    if (ok) {
      string ans;
      for (auto i : range(len)) {
        if (used[i]) {
          ans.push_back(s[i]);
        }
      }
      sort(ans.begin(), ans.end());
      printf("%s\n", ans.c_str());
      return 0;
    }
  }
  return 0;
}
