#include <bits/stdc++.h>
using namespace std;
string s;
int n, k;
struct TrieNode {
  int value;
  int sum;
  TrieNode *arr[2];
};
TrieNode *newNode() {
  TrieNode *temp = new TrieNode;
  temp->value = 0;
  temp->arr[0] = temp->arr[1] = NULL;
  return temp;
}
int memo[5100][5100];
int solve(int l, int r) {
  if (r - l + 1 <= 4) return (s[l] == s[r]);
  if (memo[l][r] != -1) return memo[l][r];
  if (s[l] == s[r]) return memo[l][r] = solve(l + 2, r - 2);
  return memo[l][r] = 0;
}
void dfs(TrieNode *u) {
  u->sum = u->value;
  if (u->arr[0] != NULL) {
    dfs(u->arr[0]);
    u->sum += u->arr[0]->sum;
  }
  if (u->arr[1] != NULL) {
    dfs(u->arr[1]);
    u->sum += u->arr[1]->sum;
  }
}
int main() {
  cin >> s;
  cin >> k;
  n = s.size();
  s = '@' + s;
  memset(memo, -1, sizeof(memo));
  TrieNode *root = newNode();
  for (int i = 1; i <= n; i++) {
    TrieNode *temp = root;
    for (int j = i; j <= n; j++) {
      int cur = s[j] - 'a';
      int val = solve(i, j);
      if (temp->arr[cur] == NULL) {
        temp->arr[cur] = newNode();
      }
      temp = temp->arr[cur];
      temp->value += val;
    }
  }
  dfs(root);
  vector<int> ans;
  int cnt = 0;
  while (k) {
    int sumL = root->value;
    if (k <= sumL) break;
    if (root->arr[0] != NULL) sumL += root->arr[0]->sum;
    if (sumL < k) {
      ans.push_back(1);
      k -= sumL;
      root = root->arr[1];
    } else {
      ans.push_back(0);
      k -= root->value;
      root = root->arr[0];
    }
  }
  for (int x : ans) {
    cout << (char)(x + 'a');
  }
}
