#include <bits/stdc++.h>
using namespace std;
const int maxn = 1010;
const int mod = 1e9 + 7;
char s[100010];
char ans[100010];
int vis[30];
set<int> st;
inline void RI(int &x) {
  char c;
  while ((c = getchar()) < '0' || c > '9')
    ;
  x = c - '0';
  while ((c = getchar()) >= '0' && c <= '9') x = (x << 3) + (x << 1) + c - '0';
}
int main() {
  int m;
  RI(m);
  cin >> s;
  int n = strlen(s);
  st.clear();
  int ma = 0;
  for (int i = 0; i <= n - m;) {
    memset(vis, -1, sizeof(vis));
    for (int j = i; j <= i + m - 1; j++) {
      vis[s[j] - 'a'] = j;
    }
    int j;
    for (j = 0; j <= 25; j++) {
      if (vis[j] != -1) {
        ma = max(ma, s[vis[j]] - 'a');
        st.insert(vis[j]);
        break;
      }
    }
    i = vis[j] + 1;
  }
  for (int i = 0; i <= n - 1; i++) {
    if (s[i] - 'a' < ma) st.insert(i);
  }
  int sum = 0;
  for (set<int>::iterator it = st.begin(); it != st.end(); it++) {
    sum++;
    ans[sum] = s[*it];
  }
  sort(ans + 1, ans + sum + 1);
  cout << ans + 1 << endl;
  return 0;
}
