#include <bits/stdc++.h>
using namespace std;
void Read(int &n) {
  int sign = 1;
  n = 0;
  char c;
  do {
    c = getchar();
  } while (!isdigit(c) && c != '-');
  if (c == '-') {
    c = getchar();
    sign = -1;
  }
  do {
    n = n * 10 + c - 48;
    c = getchar();
  } while (isdigit(c));
  n *= sign;
}
void Read(long long &n) {
  int sign = 1;
  n = 0;
  char c;
  do {
    c = getchar();
  } while (!isdigit(c) && c != '-');
  if (c == '-') {
    c = getchar();
    sign = -1;
  }
  do {
    n = n * 10ll + c - 48;
    c = getchar();
  } while (isdigit(c));
  n *= sign;
}
void Write(int n) {
  if (n == 0) {
    putchar('0');
    return;
  } else if (n < 0)
    putchar('-'), n = -n;
  char C[30];
  int d = 0;
  while (n) {
    C[++d] = (n % 10) + 48;
    n /= 10;
  }
  for (int i = d; i >= 1; --i) putchar(C[i]);
}
void Write(long long n) {
  if (n == 0) {
    putchar('0');
    return;
  } else if (n < 0)
    putchar('-'), n = -n;
  char C[30];
  int d = 0;
  while (n) {
    C[++d] = (n % 10ll) + 48;
    n /= 10ll;
  }
  for (int i = d; i >= 1; --i) putchar(C[i]);
}
const int maxn = 1e5 + 5;
char W[maxn];
int n, m;
int cnt[30];
int Get(int c) {
  int pre = 0, cur = 0, ret = 0;
  for (int i = 1; i <= n; ++i) {
    if (W[i] < 'a' + c) pre = i, cur = i;
    if (W[i] == 'a' + c) cur = i;
    if (i - pre == m) {
      if (cur > pre)
        pre = cur, ret++;
      else
        return -1;
    }
  }
  return ret;
}
int main() {
  scanf("%d", &m);
  scanf("%s", W + 1);
  n = strlen(W + 1);
  for (int i = 1; i <= n; ++i) cnt[W[i] - 'a']++;
  for (int i = 0; i <= 25; ++i) {
    int c = Get(i);
    if (c != -1) {
      for (int j = 0; j < i; ++j)
        for (int k = 1; k <= cnt[j]; ++k) putchar('a' + j);
      for (int k = 1; k <= c; ++k) putchar('a' + i);
      return 0;
    }
  }
  return 0;
}
