Substring Order 2

char str[MX];
int sa[MX], rk[MX], buckets[MX], lcp[MX], gl2[MX]; //using buckets for readability
int tmp[MX], pos[MX], st[MX * 2][18], n;
ll k, tot = 0;
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]]++);
  }
}

void rmq_precomp(){
  init(st, 63);
    gl2[1] = 0;
    for(int i = 2; i<=n; i++) gl2[i] = 1 + gl2[i/2];
  for(int i = 1; i<=n; i++) st[i][0] = lcp[i];
  for(int j = 1; (1 << j) <= n; j++){
    for(int i = 1; i <= n; i++){
      st[i][j] = min(st[i][j - 1], st[i + (1 << (j - 1))][j - 1]);
    }
  }
}

int query(int a, int b){ //max prefix amoung suffix starting at a and suffix starting at b [a, b]
    //return 0;
    a++, b++;
  if(a > b) swap(a, b);
  int k = gl2[b - a];
  return min(st[a][k], st[b - (1 << k)][k]);
}
bool add(ll val){
    if(tot + val >= k) return 1;
    tot += val;
    return 0;
}

bool check(int x, int k, int pre){
    return query(k, x) >= pre;
}

int bsrch(int l, int r, int k, int pre){
    if(r - l < 2){
        for(int i = l; i <= r; i++){
            if(!check(i, k, pre)) return i - 1;
        }
        return r;
    }
    int mid = (l + r)/2;
    if(check(mid, k, pre)){
        return bsrch(mid, r, k, pre);
    }else{
        return bsrch(l, mid-1, k, pre);
    }
}
void print(int l, int r){
    for(int i = l; i<=r; i++) cout << str[i];
    cout << "\n";
}
int main(){
  cin.tie(0) -> sync_with_stdio(0);
    cin >> (str + 1);
    cin >> k;
    n =strlen(str + 1);
    comp_SA();
    make_lcp();
    rmq_precomp();
    int p = 0;
    for(int i = 1; i<=n; i++){
        int old = p;
        for(p = max(p, lcp[i] + 1); p <= lcp[i+1]; p++){
            int v = bsrch(i+1, n, i, p) - i+1;
            if(add(v)){
                print(sa[i], sa[i] + p - 1);
                i = n+1; break;
            }
        }
        if(i > n) break;
        p = lcp[i+1];
        int dist = (n - (sa[i] + max(old, p))) + 1;
        if(add(dist)){
            print(sa[i], sa[i] + max(old, p) + k - tot - 1);
            break;
        }
    }
  return 0;
}