#include <bits/stdc++.h>
using namespace std;
vector<int> vec[200];
int segID = 0;
int segX[100010];
int segY[100010];
int used[200];
vector<int> usedLetter;
int taken[100010];
int freq[200];
bool visited[100010];
int main() {
  memset(taken, 0, sizeof(taken));
  memset(freq, 0, sizeof(freq));
  memset(used, 0, sizeof(used));
  memset(visited, false, sizeof(visited));
  int m;
  cin >> m;
  string s;
  cin >> s;
  int len = s.size();
  set<int> st;
  for (int i = 0; i < len; i++) {
    int x = s[i];
    freq[x]++;
    vec[x].push_back(i);
    if (used[x] == 0) usedLetter.push_back(x), used[x] = 1;
    if (i + m - 1 < len) {
      st.insert(i);
      segX[segID] = i;
      segY[segID] = i + m - 1;
      segID++;
    }
  }
  sort(usedLetter.begin(), usedLetter.end());
  set<int>::iterator it;
  int mxI = 0;
  for (int k = 0; k < usedLetter.size() && !st.empty(); k++) {
    if (vec[usedLetter[k]].size() > 0) {
      bool found = false;
      int i = usedLetter[k];
      int savePrvFoundID = -1;
      for (int j = 0; j < segID && !st.empty(); j++) {
        int x = segX[j];
        int y = segY[j];
        if (savePrvFoundID != -1 && savePrvFoundID >= x &&
            savePrvFoundID <= y) {
          if (!visited[x]) {
            visited[x] = true;
            mxI = max(mxI, i);
          }
        } else if (!visited[x]) {
          int idY =
              lower_bound(vec[i].begin(), vec[i].end(), y) - vec[i].begin();
          if (vec[i][idY] != y) idY--;
          if (vec[i][idY] >= x && vec[i][idY] <= y && idY >= 0) {
            visited[x] = true;
            taken[vec[i][idY]] = 1;
            mxI = max(mxI, i);
            savePrvFoundID = vec[i][idY];
          }
        }
      }
    }
  }
  for (int i = 97; i < mxI; i++) {
    for (int j = 0; j < freq[i]; j++) {
      char c = i;
      cout << c;
    }
  }
  for (int i = 0; i < len; i++) {
    int x = s[i];
    if (x == mxI && taken[i] == 1) cout << s[i];
  }
  cout << "\n";
  return 0;
}
