#include <bits/stdc++.h>
using namespace std;
const int maxn = 5005;
int n, d[maxn + 1][maxn + 1], sum[maxn + 1], mi[maxn + 1];
char s[maxn + 5];
char ans[maxn + 5];
int l = 0;
int k;
bool alive[maxn + 1];
void read() {
  scanf("%s\n", s);
  scanf("%d", &k);
  n = strlen(s);
  for (int i = n; i > 0; i--) s[i] = s[i - 1];
}
void work() {
  memset(ans, 0, sizeof(ans));
  memset(d, 0, sizeof(d));
  memset(sum, 0, sizeof(sum));
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j + i - 1 <= n; j++) {
      int l = j, r = j + i - 1;
      if (i <= 4)
        d[l][r] = (s[l] == s[r]);
      else {
        d[l][r] = (s[l] == s[r] && d[l + 2][r - 2]);
      }
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = i; j <= n; j++) {
      sum[i] += d[i][j];
    }
  }
  memset(mi, 0, sizeof(mi));
  int oa, a, ob, b;
  for (int i = 1; i <= n; i++) alive[i] = 1;
  for (int i = 1; i <= n; i++) {
    oa = a = ob = b = 0;
    for (int j = 1; j <= n - l; j++) {
      if (alive[j] == 0) continue;
      if (s[j + l] == 'a') {
        if (d[j][j + l]) {
          oa++;
          sum[j]--;
        }
        a += sum[j];
      } else {
        if (d[j][j + l]) {
          ob++;
          sum[j]--;
        }
        b += sum[j];
      }
    }
    if (oa >= k) {
      ans[l++] = 'a';
      break;
    }
    k -= oa;
    if (a >= k) {
      for (int j = 1; j <= n - l; j++) {
        if (s[j + l] == 'b') alive[j] = 0;
      }
      ans[l++] = 'a';
      continue;
    }
    k -= a;
    for (int j = 1; j <= n - l; j++) {
      if (s[j + l] == 'a') alive[j] = 0;
    }
    if (ob >= k) {
      ans[l++] = 'b';
      break;
    }
    k -= ob;
    ans[l++] = 'b';
  }
  printf("%s\n", ans);
}
int main() {
  read();
  work();
  return 0;
}
