#include <bits/stdc++.h>
using namespace std;
char s[5014], a[5014], b[5014];
int za[5014], zb[5014];
vector<int> e[5014];
void adde(int l, int r) {
  for (; l <= r; l += 2, r -= 2) {
    e[l].push_back(r);
  }
}
int nx[(5014 * 5014 / 2)][2], dp[(5014 * 5014 / 2)], spt;
int newnode() {
  for (int i = 0; i < 2; i++) nx[spt][i] = 0;
  dp[spt] = 0;
  return spt++;
}
void add(char* s, int p, int id) {
  int j = 0;
  for (int i = 0; s[i]; i++) {
    int a = s[i] - 'a';
    if (nx[p][a] == 0) nx[p][a] = newnode();
    p = nx[p][a];
    if (j < ((int)e[id].size()) && e[id][j] == id + i) {
      dp[p]++;
      j++;
    }
  }
  assert(j == ((int)e[id].size()));
  e[id].clear();
}
void make_dp(int p) {
  if (nx[p][0]) {
    make_dp(nx[p][0]);
    dp[p] += dp[nx[p][0]];
  }
  if (nx[p][1]) {
    make_dp(nx[p][1]);
    dp[p] += dp[nx[p][1]];
  }
}
int l;
void test(int x, int y) {
  for (; x >= 0 && y < l && s[x] == s[y]; x -= 2, y += 2)
    ;
  x += 2;
  y -= 2;
  adde(x, y);
}
long long int k;
char aa[5014];
int at;
int sol(int uuu) {
  l = 0;
  for (; s[l]; l++)
    ;
  for (int i = 0; i < l; i++) {
    if (i + 0 < l) test(i, i + 0);
    if (i + 1 < l) test(i, i + 1);
    if (i + 2 < l) test(i, i + 2);
    if (i + 3 < l) test(i, i + 3);
  }
  spt = 1;
  int root = newnode();
  for (int i = 0; i < l; i++) {
    add(s + i, root, i);
  }
  dp[0] = 0;
  make_dp(root);
  int p = root;
  at = 0;
  for (; k;) {
    int ll = nx[p][0];
    int rr = nx[p][1];
    if (k <= dp[p] - dp[rr] - dp[ll]) {
      aa[at] = 0;
      puts(aa);
      break;
    }
    k -= dp[p] - dp[rr] - dp[ll];
    if (dp[ll] >= k) {
      p = ll;
      aa[at++] = 'a';
    } else {
      k -= dp[ll];
      p = rr;
      aa[at++] = 'b';
    }
  }
  return 1;
}
int main() {
  while (scanf("%s %I64d", s, &k) != EOF) sol(0);
  int t;
  if (scanf("%d", &(t)) != EOF) {
    for (int ti = 1; ti <= t; ti++) sol(ti);
  }
  return 0;
}
