#include <bits/stdc++.h>
using namespace std;
const int N = 5002;
int cnt[N];
int is_pal[N][N];
struct kmp {
  string s;
  vector<int> phi;
  kmp() { s = "$", phi.assign(1, 0); }
  void add(char c) {
    int n = s.size();
    s[n - 1] = c;
    phi[n - 1] = n > 1 ? phi[n - 2] : 0;
    while (phi[n - 1] and s[phi[n - 1]] != c) phi[n - 1] = phi[phi[n - 1] - 1];
    if (n > 1 and s[phi[n - 1]] == c) phi[n - 1]++;
    s += "$";
    phi.push_back(0);
  }
  int nxt(int m, char c) {
    while (m and s[m] != c) m = phi[m - 1];
    if (s[m] == c) m++;
    return m;
  }
};
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  string s;
  int k;
  cin >> s >> k;
  int n = s.size();
  for (int k = 0; k < 4; ++k) {
    for (int i = 0; i < n; ++i) {
      for (int j = 0; j < n; j += 2) {
        if (i - j < 0 or i + k + j > n or s[i - j] != s[i + k + j]) break;
        cnt[i - j]++;
        is_pal[i - j][(i + k + j) - (i - j) + 1] = 1;
      }
    }
  }
  string t = "";
  int sz = 0;
  kmp pre = kmp();
  while (k) {
    int cnta = 0, cntb = 0;
    int m = 0;
    int tot = 0;
    for (int i = 0; i < n; ++i) {
      if (m == sz) {
        cnta += cnt[i - sz] * (s[i] == 'a');
        cntb += cnt[i - sz] * (s[i] == 'b');
        if (is_pal[i - sz][sz]) tot++;
      }
      m = pre.nxt(m, s[i]);
    }
    if (m == sz and is_pal[n - sz][sz]) tot++;
    if (k <= tot)
      break;
    else
      k -= tot;
    if (k <= cnta) {
      t += 'a';
      pre.add('a');
    } else {
      k -= cnta;
      t += 'b';
      pre.add('b');
    }
    sz++;
    for (int i = 0; i < n; ++i) {
      if (is_pal[i][sz]) cnt[i]--;
    }
  }
  cout << t << endl;
}
