#include <bits/stdc++.h>
using namespace std;
const long double eps = 1e-9;
const int inf = (1 << 30) - 1;
const long long inf64 = ((long long)1 << 62) - 1;
const long double pi = acos(-1);
template <class T>
T sqr(T x) {
  return x * x;
}
template <class T>
T abs(T x) {
  return x < 0 ? -x : x;
}
const int MAXN = 120 * 1000;
int a[MAXN];
int last[MAXN][26];
int num[26];
int main() {
  int m;
  scanf("%d", &m);
  string s = "";
  while (s == "") {
    getline(cin, s);
  }
  int n = ((int)(s).size());
  for (int i = 0; i < n; ++i) {
    a[i] = s[i] - 'a';
  }
  for (int i = 0; i < 26; ++i) {
    last[0][i] = -inf;
  }
  last[0][a[0]] = 0;
  for (int i = 1; i < n; ++i) {
    for (int j = 0; j < 26; ++j) {
      last[i][j] = last[i - 1][j];
    }
    last[i][a[i]] = i;
  }
  for (int i = 0; i < 26; ++i) {
    num[i] = 0;
  }
  for (int i = 0; i < n; ++i) {
    num[a[i]]++;
  }
  for (int i = 0; i < 26; ++i) {
    int cur = 0;
    int numThis = 0;
    bool key = true;
    for (int j = 0; j < n; ++j) {
      cur++;
      if (a[j] < i) {
        cur = 0;
      }
      if (cur == m) {
        cur = j - last[j][i];
        numThis++;
        if (cur >= m) {
          key = false;
          break;
        }
      }
    }
    if (key) {
      for (int j = 0; j < i; ++j) {
        for (int k = 0; k < num[j]; ++k) {
          printf("%c", 'a' + j);
        }
      }
      for (int j = 0; j < numThis; ++j) {
        printf("%c", 'a' + i);
      }
      printf("\n");
      return 0;
    }
  }
  return 0;
}
