Substring Order 1

char str[MX];
int sa[MX], rk[MX], buckets[MX], lcp[MX], gl2[MX];
int tmp[MX], pos[MX], st[MX * 2][40], n;

void comp_SA(){
  int mx = 260;
  for(int i = 1; i<=n; i++){
    sa[i] = i;
    rk[i] = str[i];
  }

  for(int i = 1; i >> 1 < n; i <<= 1){
    int k = i >> 1, p = k;
    for(int j = 1; j<=k; j++) buckets[j] = n - (j - 1);
    for(int j = 1; j<=n; j++) if(sa[j] > k) buckets[++p] = sa[j] - k;
    for(int j = 1; j<=n; j++) pos[j] = 0;
    for(int j = 1; j<=n; j++) pos[rk[j]]++;
    for(int j = 1; j<=mx; j++) pos[j] += pos[j - 1];
    for(int j = n; j; j--) sa[pos[rk[buckets[j]]]--] = buckets[j];
    for(int j = 1; j<=n; j++) tmp[sa[j]] = tmp[sa[j - 1]] + (rk[sa[j]] != rk[sa[j - 1]] || rk[sa[j] + k] != rk[sa[j - 1] + k]);
    for(int j = 1; j<=n; j++) rk[j] = tmp[j];
    mx = rk[sa[n]];
    if(mx == n) break;
  }

}

void make_lcp(){
  for(int i = 1; i<=n; i++){
        if(rk[i] == 1) cont;
    lcp[rk[i]] = max(0, lcp[rk[i - 1]] - 1);
    for(; str[i + lcp[rk[i]]] == str[sa[rk[i] - 1] + lcp[rk[i]]]; lcp[rk[i]]++); /
  }
}

int main(){
  cin.tie(0) -> sync_with_stdio(0);
    cin >> (str + 1);
    n = strlen(str + 1);
    ll k; cin >> k;
    comp_SA();
    make_lcp();
    for(int i = 1; i<=n; i++){
        int dist = (n - (sa[i] + lcp[i])) + 1;
        if(dist < k){
            k -= dist;
        }else{
            for(int j = sa[i]; j < sa[i] + lcp[i] + k; j++){
                cout << str[j];
            }
            cout << "\n";
            break;
        }
    }
  return 0;
}