#include <bits/stdc++.h>
using namespace std;
int xx[4] = {0, 0, 1, -1};
int yy[4] = {1, -1, 0, 0};
int n;
string s;
int k;
struct trie {
  int k;
  int next[2];
  trie() { memset(next, 0, sizeof(next)); }
};
vector<trie> a;
bool good[int(5010)][int(5010)];
int f[int(5010) * 2500][3];
void push(int l) {
  int node = 0;
  for (int i = (l), _b = (s.length() - 1); i <= _b; i++) {
    if (a[node].next[s[i] - 'a'] == 0) {
      a[node].next[s[i] - 'a'] = a.size();
      a.push_back(trie());
    }
    node = a[node].next[s[i] - 'a'];
    if (good[l][i]) f[node][2] += 1;
  }
}
void dfs(int x) {
  for (int i = (0), _b = (1); i <= _b; i++) {
    int y = a[x].next[i];
    if (y == 0) continue;
    dfs(y);
    f[x][i] += f[y][0] + f[y][1] + f[y][2];
  }
}
string res;
void find_res(int x) {
  if (f[x][2] >= k) {
    return;
  } else {
    k -= f[x][2];
    if (f[x][0] >= k) {
      res += 'a';
      find_res(a[x].next[0]);
    } else {
      res += 'b';
      k -= f[x][0];
      find_res(a[x].next[1]);
    }
  }
}
int main() {
  cin >> s;
  cin >> k;
  for (int i = (s.length() - 1), _a = (0); i >= _a; i--) {
    for (int j = (i), _b = (s.length() - 1); j <= _b; j++) {
      int u = i + 2;
      int v = j - 2;
      if (s[i] == s[j] && (u >= v || good[u][v])) good[i][j] = true;
    }
  }
  a.push_back(trie());
  for (int i = (0), _b = (s.length() - 1); i <= _b; i++) {
    push(i);
  }
  dfs(0);
  find_res(0);
  cout << res << endl;
}
