#include <bits/stdc++.h>
using namespace std;
int dp[5001][5001];
string str = "*", buf, ans = "";
int k;
bool solve(int l, int r) {
  if (l > r) return 1;
  if (dp[l][r] == -1) {
    if (str[l] == str[r])
      return dp[l][r] = solve(l + 2, r - 2);
    else
      return dp[l][r] = 0;
  }
  return dp[l][r];
}
struct node {
  int cnt = 0;
  node *child[2];
} * head;
void init() { head = new node(); }
void update(string word, int l) {
  node *cur = head;
  for (int i = 0; i < word.size(); i++) {
    int c = word[i] - 'a';
    if (cur->child[c] == NULL) cur->child[c] = new node();
    cur = cur->child[c];
    cur->cnt += dp[l][l + i];
  }
}
void DFS(node *cur, bool b) {
  k -= cur->cnt;
  if (k >= 1) {
    for (int i = 0; i < 2; i++)
      if (cur->child[i] != NULL && k >= 1) DFS(cur->child[i], i);
  }
  if (k <= 0) ans += char(b + 'a');
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  memset(dp, -1, sizeof(dp));
  cin >> buf >> k;
  str += buf;
  init();
  for (int i = 1; i < str.size(); i++)
    for (int j = i; j < str.size(); j++) solve(i, j);
  for (int i = 1; i < str.size(); i++) update(str.substr(i, str.size() - i), i);
  DFS(head, 0);
  ans.resize(ans.size() - 1);
  reverse(ans.begin(), ans.end());
  cout << ans;
  return 0;
}
