#include <bits/stdc++.h>
template <typename A, typename B>
inline char SMAX(A &a, const B &b) {
  return a < b ? a = b, 1 : 0;
}
template <typename A, typename B>
inline char SMIN(A &a, const B &b) {
  return b < a ? a = b, 1 : 0;
}
template <typename I>
inline void read(I &x) {
  int f = 0, c;
  while (!isdigit(c = getchar())) c == '-' ? f = 1 : 0;
  x = c & 15;
  while (isdigit(c = getchar())) x = (x << 1) + (x << 3) + (c & 15);
  f ? x = -x : 0;
}
const int N = 5000 + 7;
int n, k, dp[N][N];
int sa[N], rk[N], sc[N], tax[N], h[N];
char s[N];
inline void DP() {
  for (int i = 1; i <= n; ++i) dp[i][i] = 1;
  for (int i = n; i; --i)
    for (int j = i + 1; j <= n; ++j) {
      if (j - i >= 4 && s[i] == s[j])
        dp[i][j] = dp[i + 2][j - 2];
      else if (j - i == 3)
        dp[i][j] = s[i] == s[j];
      else if (j - i == 2)
        dp[i][j] = s[i] == s[j];
      else if (j - i == 1)
        dp[i][j] = s[i] == s[j];
    }
}
inline void Make_SA() {
  int m = 'b', p, *rnk = rk, *sec = sc;
  for (int i = 1; i <= m; ++i) tax[i] = 0;
  for (int i = 1; i <= n; ++i) ++tax[rnk[i] = s[i]];
  for (int i = 1; i <= m; ++i) tax[i] += tax[i - 1];
  for (int i = n; i; --i) sa[tax[rnk[i]]--] = i;
  for (int k = 1; k <= n; k <<= 1) {
    p = 0;
    for (int i = n - k + 1; i <= n; ++i) sec[++p] = i;
    for (int i = 1; i <= n; ++i)
      if (sa[i] > k) sec[++p] = sa[i] - k;
    for (int i = 1; i <= m; ++i) tax[i] = 0;
    for (int i = 1; i <= n; ++i) ++tax[rnk[sec[i]]];
    for (int i = 1; i <= m; ++i) tax[i] += tax[i - 1];
    for (int i = n; i; --i) sa[tax[rnk[sec[i]]]--] = sec[i];
    std::swap(rnk, sec);
    rnk[sa[1]] = p = 1;
    for (int i = 2; i <= n; ++i)
      rnk[sa[i]] =
          (sec[sa[i]] == sec[sa[i - 1]] && sec[sa[i] + k] == sec[sa[i - 1] + k]
               ? p
               : ++p);
    if (p >= n)
      break;
    else
      m = p;
  }
  for (int i = 1; i <= n; ++i) rk[sa[i]] = i;
}
inline void Make_height() {
  for (int i = 1, f = 0; i <= n; ++i) {
    if (f > 0) --f;
    int j = sa[rk[i] - 1];
    while (i + f <= n && j + f <= n && s[i + f] == s[j + f]) ++f;
    h[rk[i]] = f;
  }
}
inline int Count(int l, int r) {
  int lp = rk[l], rp = lp;
  for (; h[lp] >= r - l + 1; --lp)
    ;
  for (; h[rp + 1] >= r - l + 1; ++rp)
    ;
  return rp - lp + 1;
}
inline void Print(int l, int r) {
  for (int i = l; i <= r; ++i) putchar(s[i]);
  putchar('\n');
  exit(0);
}
int main() {
  scanf("%s%d", s + 1, &k);
  n = strlen(s + 1);
  DP();
  Make_SA();
  Make_height();
  for (int i = 1; i <= n; ++i)
    for (int j = sa[i] + h[i]; j <= n; ++j)
      if (dp[sa[i]][j] && (k -= Count(sa[i], j)) <= 0) Print(sa[i], j);
}
