#include <bits/stdc++.h>
using namespace std;
const int INF = (1LL << 30) - 1;
const long long int LINF = (1LL << 62) - 1;
const int NMAX = 5000;
struct node {
  node *f[2];
  int cnt;
  int w;
  node() {
    f[0] = f[1] = NULL;
    cnt = w = 0;
  }
};
int N, K;
char S[NMAX + 5];
bool P[NMAX + 5][NMAX + 5];
node *root;
void dfs(node *Q) {
  int a = 0, b = 0;
  if (Q->f[0]) {
    dfs(Q->f[0]);
    a = Q->f[0]->cnt;
  }
  if (Q->f[1]) {
    dfs(Q->f[1]);
    b = Q->f[1]->cnt;
  }
  Q->cnt = Q->w + a + b;
}
void search(node *Q, int K) {
  if (!K) return;
  if (K <= Q->w) return;
  K -= Q->w;
  if (Q->f[0] && Q->f[0]->cnt >= K) {
    printf("a");
    search(Q->f[0], K);
  } else if (Q->f[1]) {
    printf("b");
    search(Q->f[1], K - ((Q->f[0] != NULL) ? Q->f[0]->cnt : 0));
  }
}
int main() {
  int i, j, k, c;
  node *Q;
  cin.sync_with_stdio(false);
  scanf("%s", S + 1);
  scanf("%d", &K);
  N = strlen(S + 1);
  for (k = 1; k <= N; k++) {
    for (i = k, j = k; i >= 1 && j <= N; i -= 2, j += 2)
      if (S[i] != S[j])
        break;
      else
        P[i][j] = 1;
    for (i = k - 1, j = k + 1; i >= 1 && j <= N; i -= 2, j += 2)
      if (S[i] != S[j])
        break;
      else
        P[i][j] = 1;
    for (i = k, j = k + 1; i >= 1 && j <= N; i -= 2, j += 2)
      if (S[i] != S[j])
        break;
      else
        P[i][j] = 1;
    for (i = k - 1, j = k + 2; i >= 1 && j <= N; i -= 2, j += 2)
      if (S[i] != S[j])
        break;
      else
        P[i][j] = 1;
  }
  root = new node();
  for (i = 1; i <= N; i++) {
    Q = root;
    for (j = i; j <= N; j++) {
      c = S[j] - 'a';
      if (Q->f[c] == NULL) Q->f[c] = new node();
      Q = Q->f[c];
      Q->w += P[i][j];
    }
  }
  dfs(root);
  search(root, K);
  return 0;
}
