#include <bits/stdc++.h>
using namespace std;
const int N = 5010;
bool dp[N][N];
int sum[N];
char s[N];
int main() {
  scanf("%s", s + 1);
  int n = strlen(s + 1);
  for (int i = 1; i <= n; i++) dp[i][i] = true;
  for (int i = 2; i <= n; i++) {
    for (int j = 1; j + i - 1 <= n; j++) {
      int y = j + i - 1;
      if (s[j] != s[y]) continue;
      if (i == 4) {
        if (s[j] == s[y]) dp[j][y] = true;
        continue;
      }
      if (i == 5) {
        if (s[j] == s[j + 4]) dp[j][y] = true;
        continue;
      }
      if (i <= 3 || dp[j + 2][y - 2]) dp[j][y] = true;
    }
  }
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= n; j++) sum[i] += dp[i][j];
  int m;
  scanf("%d", &m);
  int q[N], tq[N];
  for (int i = 1; i <= n; i++) q[i] = i;
  int sz = n;
  for (int i = 1; i <= n && m; i++) {
    for (char c = 'a'; c <= 'z'; c++) {
      int tot = 0, cc = 0;
      for (int j = 1; j <= sz; j++)
        if (s[q[j] + i - 1] == c) {
          tot += sum[q[j]];
          if (dp[q[j]][q[j] + i - 1]) cc++, sum[q[j]]--;
        }
      if (tot < m) {
        m -= tot;
        continue;
      }
      printf("%c", c);
      if (cc >= m) {
        printf("\n");
        return 0;
      }
      m -= cc;
      int t = 0;
      for (int j = 1; j <= sz; j++)
        if (s[q[j] + i - 1] == c && q[j] + i <= n) {
          q[++t] = q[j];
        }
      sz = t;
      break;
    }
  }
  return 0;
}
