#include <bits/stdc++.h>
using namespace std;
struct node {
  int sum, cnt;
  node *a[2];
  node() {
    sum = cnt = 0;
    a[0] = a[1] = 0;
  }
};
const int nm = 5002;
char s[nm];
int n, k;
node *root;
bool ok[nm][nm], ok2[nm][nm];
char kq[nm];
int nkq;
void them(int i) {
  int last = i - 1;
  node *p = root;
  for (int j = i; j < n; ++j) {
    if (ok[i][j]) {
      for (int k = last + 1; k <= j; ++k) {
        if (!p->a[s[k] - 'a']) p->a[s[k] - 'a'] = new node();
        p = p->a[s[k] - 'a'];
      }
      p->cnt++;
      last = j;
    }
  }
}
void dfs(node *p) {
  if (!p) return;
  dfs(p->a[0]);
  dfs(p->a[1]);
  p->sum = p->cnt;
  if (p->a[0]) p->sum += p->a[0]->sum;
  if (p->a[1]) p->sum += p->a[1]->sum;
}
void dfs2(node *p) {
  if (p->cnt >= k) return;
  k -= p->cnt;
  if (p->a[0] && p->a[0]->sum >= k) {
    kq[nkq++] = 'a';
    dfs2(p->a[0]);
  } else {
    if (p->a[0]) k -= p->a[0]->sum;
    kq[nkq++] = 'b';
    dfs2(p->a[1]);
  }
}
int main() {
  scanf("%s%d", s, &k);
  n = strlen(s);
  for (int i = 0; i < n; ++i) ok[i][i] = ok2[i][i] = 1;
  for (int i = 0; i + 1 < n; ++i) {
    ok[i][i + 1] = (s[i] == s[i + 1]);
    ok2[i][i + 1] = 1;
  }
  for (int l = 3; l <= n; ++l) {
    for (int i = 0; i + l - 1 < n; ++i) {
      int j = i + l - 1;
      ok[i][j] = ok2[i + 1][j - 1] && s[i] == s[j];
      ok2[i][j] = ok[i + 1][j - 1];
    }
  }
  root = new node();
  for (int i = 0; i < n; ++i) them(i);
  dfs(root);
  dfs2(root);
  printf("%s\n", kq);
}
