#include <bits/stdc++.h>
using namespace std;
const int N = 5050;
int k, len;
char s[N];
bool can[N][N];
vector<char> ans;
struct node {
  node *l, *r;
  int cnt;
};
inline int getCnt(node* root) {
  if (!root) {
    return 0;
  } else {
    return root->cnt;
  }
}
inline void go(int l, int r) {
  while (true) {
    if (l < 1 || r > len) {
      break;
    }
    if (s[l] != s[r]) {
      break;
    }
    can[l][r] = true;
    l -= 2;
    r += 2;
  }
}
inline void printAns() {
  ans.pop_back();
  for (size_t i = 0; i < ans.size(); i++) {
    printf("%c", ans[i]);
  }
  puts("");
  exit(0);
}
void dfs(node* root) {
  if (root->l) {
    dfs(root->l);
    root->cnt += getCnt(root->l);
  }
  if (root->r) {
    dfs(root->r);
    root->cnt += getCnt(root->r);
  }
}
void dfs(node* root, int k) {
  if (k <= 0) {
    printAns();
  }
  int le = getCnt(root->l);
  int ri = getCnt(root->r);
  int here = root->cnt - le - ri;
  if (le >= k - here) {
    ans.push_back('a');
    dfs(root->l, k - here);
  } else {
    ans.push_back('b');
    dfs(root->r, k - here - le);
  }
}
int main() {
  gets(s + 1);
  len = strlen(s + 1);
  scanf("%d", &k);
  for (int i = 1; i <= len; i++) {
    can[i][i] = true;
    go(i, i + 1);
    go(i - 1, i + 1);
    go(i - 2, i + 2);
    go(i - 2, i + 1);
    go(i - 1, i + 2);
  }
  node *root = new node(), *cur;
  for (int i = 1; i <= len; i++) {
    cur = root;
    for (int j = i; j <= len; j++) {
      if (s[j] == 'a' && cur->l == NULL) {
        cur->l = new node();
      }
      if (s[j] == 'b' && cur->r == NULL) {
        cur->r = new node();
      }
      if (s[j] == 'a') {
        cur = cur->l;
      } else {
        cur = cur->r;
      }
      if (can[i][j]) {
        cur->cnt++;
      }
    }
  }
  dfs(root);
  dfs(root, k);
  return 0;
}
