#include <bits/stdc++.h>
using namespace std;
const int MAX = 5001;
struct nod {
  int val, left, right;
};
vector<bitset<MAX> > dp(MAX);
vector<nod> arb;
char s[MAX], sol[MAX];
int k, n;
void makedp() {
  for (int lg = 1; lg <= n; lg++)
    for (int i = 1; i <= n + 1 - lg; i++)
      if (lg == 1 or (lg <= 4 and s[i] == s[i + lg - 1]) or
          (lg > 4 and dp[i + 2][i + lg - 3] and s[i] == s[i + lg - 1]))
        dp[i][i + lg - 1] = 1;
}
void preord(int cur, int vf) {
  if (k > 0 and arb[cur].val >= k) sol[vf] = '\0';
  k -= arb[cur].val;
  if (arb[cur].left != -1) {
    if (k > 0) sol[vf] = 'a';
    preord(arb[cur].left, vf + 1);
  }
  if (arb[cur].right != -1) {
    if (k > 0) sol[vf] = 'b';
    preord(arb[cur].right, vf + 1);
  }
}
int main() {
  cin >> s + 1 >> k;
  nod rad = {0, -1, -1};
  arb.push_back(rad);
  for (n = 1; s[n + 1] != '\0'; n++)
    ;
  makedp();
  for (int i = 1; i <= n; i++) {
    int cur = 0;
    for (int j = i; j <= n; j++) {
      if (s[j] == 'a') {
        if (arb[cur].left == -1) {
          arb[cur].left = arb.size();
          nod nou = {0, -1, -1};
          arb.push_back(nou);
        }
        cur = arb[cur].left;
      } else {
        if (arb[cur].right == -1) {
          arb[cur].right = arb.size();
          nod nou = {0, -1, -1};
          arb.push_back(nou);
        }
        cur = arb[cur].right;
      }
      if (dp[i][j]) arb[cur].val++;
    }
  }
  preord(0, 0);
  cout << sol;
  return 0;
}
