#include <bits/stdc++.h>
using namespace std;
char s[1 << 20] = {0};
int main() {
  int m, cnt[26] = {0};
  scanf("%d%s", &m, s);
  int n = strlen(s);
  vector<vector<int>> p(26);
  for (int i = n - 1; i >= 0; i--) p[s[i] - 'a'].push_back(i);
  set<int> S;
  S.insert(-1);
  S.insert(n);
  for (int k = 0; k < 26; k++) {
    auto it = S.begin(), jt = ++S.begin();
    for (int i = p[k].size() - 1; i >= 0 && *it < n; it = jt++) {
      if (*jt - *it > m) {
        for (; i >= 0 && p[k][i] <= *it; i--)
          ;
        for (; i > 0 && p[k][i - 1] <= *it + m; i--)
          ;
        if (i >= 0 && *it < p[k][i] && p[k][i] < *jt) {
          cnt[k]++;
          jt = S.insert(p[k][i]).first;
        }
      }
    }
  }
  for (int i = 25; i >= 0; i--)
    if (cnt[i] > 0) {
      for (int j = i - 1; j >= 0; j--) cnt[j] = p[j].size();
      break;
    }
  for (int i = 0; i < 26; i++)
    for (; cnt[i] > 0; cnt[i]--) putchar('a' + i);
  putchar('\n');
}
