#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const long long INFLL = 0x3f3f3f3f3f3f3f3fLL;
inline long long read() {
  long long x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
const int maxn = 1e5 + 10;
int m, f[30];
char s[maxn];
int main() {
  cin >> m >> s + 1;
  int n = strlen(s + 1);
  for (char c = 'a'; c <= 'z'; c++) {
    int tmp = 0, flag = 1;
    for (int i = 1; i <= n; i++) {
      tmp++;
      if (tmp > m) {
        flag = 0;
        break;
      } else if (s[i] <= c)
        tmp = 0;
    }
    if (!flag) continue;
    tmp = 0;
    for (int i = 1; i <= n; i++) {
      tmp++;
      if (s[i] < c)
        f[s[i] - 'a']++, tmp = 0;
      else if (s[i] == c) {
        int j = 1, flag = 0;
        for (; j <= m - tmp; j++) {
          if (s[i + j] <= c) {
            flag = 1;
            break;
          }
        }
        if (!flag) f[s[i] - 'a']++, tmp = 0;
      }
    }
    break;
  }
  for (char c = 'a'; c <= 'z'; c++) {
    for (int i = 0; i < f[c - 'a']; i++) putchar(c);
  }
  puts("");
  return 0;
}
