#include <bits/stdc++.h>
using namespace std;
const int N = 5005;
bool pal[N][N];
string s;
int n, k, max_val[N];
void check(int l, int len) {
  if (pal[l][l + len - 1]) {
    max_val[l]--;
    if (--k == 0) {
      cout << s.substr(l, len) << '\n';
      exit(0);
    }
  }
}
void bucket_sort(vector<int> &arr, int len) {
  if (arr[0] + len > n) return;
  vector<int> zero, one;
  int sum_zero = 0;
  for (int i = 0; i < arr.size(); ++i) {
    int start = arr[i];
    if (start + len > n) break;
    if (s[start + len - 1] == 'a') {
      sum_zero += max_val[start];
      zero.push_back(start);
    } else {
      one.push_back(start);
    }
  }
  if (sum_zero >= k) {
    for (int i = 0; i < zero.size(); ++i) check(zero[i], len);
    if (!zero.empty()) bucket_sort(zero, len + 1);
  } else {
    k -= sum_zero;
  }
  for (int i = 0; i < one.size(); ++i) check(one[i], len);
  if (!one.empty()) bucket_sort(one, len + 1);
}
int main() {
  cin >> s >> k;
  n = s.size();
  for (int i = 0; i < n; ++i) {
    pal[i][i] = true;
    if (i < n - 1 && s[i] == s[i + 1]) pal[i][i + 1] = true;
    if (i < n - 2 && s[i] == s[i + 2]) pal[i][i + 2] = true;
    if (i < n - 3 && s[i] == s[i + 3]) pal[i][i + 3] = true;
    if (i < n - 4 && s[i] == s[i + 4]) pal[i][i + 4] = true;
  }
  for (int len = 3; len <= n; ++len) {
    for (int i = 0; i <= n - len; ++i) {
      pal[i][i + len - 1] =
          max(pal[i][i + len - 1],
              pal[i + 2][i + len - 3] && s[i] == s[i + len - 1]);
    }
  }
  for (int i = 0; i < n; ++i)
    for (int j = i; j < n; ++j)
      if (pal[i][j]) max_val[i]++;
  vector<int> start;
  for (int i = 0; i < n; ++i) start.push_back(i);
  bucket_sort(start, 1);
};
