#include <bits/stdc++.h>
using namespace std;
const int N = 5050;
string s;
int n, k, dp[N][N];
bool b[N][N], c[N], nxt[2][N];
vector<char> v;
int main() {
  if (fopen("E.in", "r")) freopen("E.in", "r", stdin);
  cin >> s >> k;
  n = s.length();
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n - i; j++) {
      b[j][j + i] =
          (j + 2 > i + j - 2 || (b[j + 2][i + j - 2])) && s[j] == s[j + i];
    }
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (b[i][j]) {
        dp[i][i]++;
        dp[i][j + 1]--;
      }
    }
    int cnt = 0;
    for (int j = 0; j < n; j++) {
      cnt += dp[i][j];
      dp[i][j] = cnt;
    }
  }
  for (int i = 0; i <= n; i++) c[i] = 1;
  int cur = 0;
  for (int i = 0; i < n; i++) {
    int f[2], g[2];
    f[0] = f[1] = g[0] = g[1] = 0;
    for (int j = 1; j <= n; j++) {
      if (!c[j - 1]) continue;
      f[s[j - 1] - 'a'] += dp[j - i - 1][j - 1];
      g[s[j - 1] - 'a'] += b[j - i - 1][j - 1];
      nxt[s[j - 1] - 'a'][j] = 1;
    }
    if (cur + f[0] >= k) {
      v.push_back('a');
      if (cur + g[0] >= k) break;
      cur += g[0];
      for (int j = 0; j <= n; j++) c[j] = nxt[0][j];
    } else {
      v.push_back('b');
      if (cur + f[0] + g[1] >= k) break;
      cur += f[0] + g[1];
      for (int j = 0; j <= n; j++) c[j] = nxt[1][j];
    }
    memset(nxt, 0, sizeof(nxt));
  }
  for (int i = 0; i < v.size(); i++) {
    cout << v[i];
  }
  return 0;
}
