#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC optimize(3)
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC target("sse3","sse2","sse")
#pragma GCC target("avx","sse4","sse4.1","sse4.2","ssse3")
#pragma GCC target("f16c")
#pragma GCC optimize("inline","fast-math","unroll-loops","no-stack-protector")
#pragma GCC diagnostic error "-fwhole-program"
#pragma GCC diagnostic error "-fcse-skip-blocks"
#pragma GCC diagnostic error "-funsafe-loop-optimizations"
#pragma GCC diagnostic error "-std=c++14"
#include "bits/stdc++.h"
//#include "ext/pb_ds/tree_policy.hpp"
//#include "ext/pb_ds/assoc_container.hpp"
#define PB push_back
#define PF push_front
#define LB lower_bound
#define UB upper_bound
#define fr(x) freopen(x,"r",stdin)
#define fw(x) freopen(x,"w",stdout)
#define iout(x) printf("%d\n",x)
#define lout(x) printf("%lld\n",x)
#define REP(x,l,u) for(ll x = l;x<u;x++)
#define RREP(x,l,u) for(ll x = l;x>=u;x--)
#define complete_unique(a) a.erase(unique(a.begin(),a.end()),a.end())
#define mst(x,a) memset(x,a,sizeof(x))
#define all(a) a.begin(),a.end()
#define PII pair<int,int>
#define PLL pair<ll,ll>
#define MP make_pair
#define sqr(x) ((x)*(x))
#define lowbit(x) (x&(-x))
#define lson (ind<<1)
#define rson (ind<<1|1)
#define se second
#define fi first
#define dbg(x) cerr<<#x<<" = "<<(x)<<endl;
#define sz(x) ((int)x.size())
#define EX0 exit(0);

typedef  long long ll;
typedef unsigned long long ull;
typedef double db;
typedef long double ld;
using namespace std;
typedef vector<ll> VLL;
typedef vector<int> VI;
const int block_size = 320;
typedef complex<ll> point;
const ll mod = 1e9+7;
const ll inf = 1e9+7;
const ld eps = 1e-9;
const db PI = atan(1)*4;
template<typename T>
inline int sign(const T&a) {
    if(a<0)return -1;
    if(a>0)return 1;
    return 0;
}

template<typename T,typename S>inline bool upmin(T&a,const S&b){return a>b?a=b,1:0;}
template<typename T,typename S>inline bool upmax(T&a,const S&b){return a<b?a=b,1:0;}

template<typename T> inline void in(T &x) {
    x = 0;
    T f = 1;
    char ch = getchar();
    while (!isdigit(ch)) {
        if (ch == '-') f = -1;
        ch = getchar();
    }
    while (isdigit(ch))  {
        x = x * 10 + ch - '0';
        ch = getchar();
    }
    x *= f;
}

ll twop(int x) {
    return 1LL<<x;
}

template<typename A,typename B > inline void in(A&x,B&y) {
    in(x);
    in(y);
}
template<typename A,typename B,typename C>inline void in(A&x,B&y,C&z) {
    in(x);
    in(y);
    in(z);
}
template<typename A,typename B,typename C,typename D> inline void in(A&x,B&y,C&z,D&d) {
    in(x);
    in(y);
    in(z);
    in(d);
}
ll gcd(ll a,ll b){return b==0?a:gcd(b,a%b);}
namespace SOLVE {
    void main(){
    }
}
template <typename T>
class SuffixArray {
public:
    void build_SA(const T&str,vector<int>& sa,vector<int>& rk){
        
        const int len = sz(str);
        vector<pair<PII,int>> ord[2] = {vector<pair<PII,int>>(len),vector<pair<PII,int>>(len)};
        sa = std::vector<int>(len);
        rk = std::vector<int>(len);
        REP(i,0,len){
            ord[1][i] = MP(MP(str[i],0),i);
        }
        for(int b = 0;;b++){
            int cur = b%2;
            sort(all(ord[cur^1]));
            REP(i,0,len){
                rk[ord[cur^1][i].se] = i == 0?0:(rk[ord[cur^1][i-1].se] + (ord[cur^1][i-1].fi != ord[cur^1][i].fi));
            }
            if(rk[ord[cur^1][len-1].se] == len-1)break;
            REP(i,0,len)ord[cur][i] = MP(MP(rk[i],(i+twop(b) < len)?rk[i+twop(b)]:-inf),i);
        }
        REP(i,0,len)sa[rk[i]] = i;
    }
    vector<vector<int>>st;
    void build_height(const T&str,const vector<int>& sa,const vector<int>& rk,vector<int>&height){
        height = std::vector<int>(sz(str),0);
        const int len = sz(str);
        REP(i,0,len){
            if(rk[i] == 0)continue;
            while(i+height[rk[i]] < len && sa[rk[i]-1] + height[rk[i]] < len && str[i+height[rk[i]]] == str[sa[rk[i]-1] + height[rk[i]]]){
                height[rk[i]]++;
            }
            if(i!=len-1){
                height[rk[i+1]] = max(0,height[rk[i]]-1);
            }
        }
    }
    
    T str;
    vector<int>sa,rk;
    void init(const T&s,bool getHeight = 0){
        str = s;
        build_SA(str, sa, rk);
        if(getHeight){
            st = vector<vector<int>>(ceil(log2(sz(s))+eps),vector<int>(sz(s)));
            build_height(str, sa, rk, st[0]);
            REP(i,0,sz(st)-1){
                REP(j,0,sz(s)-twop(i)){
                    st[i+1][j] = min(st[i][j],st[i][j+twop(i)]);
                }
            }
        }
    }
    
    void test(){
        REP(i,0,sz(str)){
            cout<<(i)<<" ";
            REP(j,sa[i],sz(str)){if(str[j]<0)break;cout<<char(str[j]);}
            cout<<endl;
        }
    }
    
    int getLCP(int l,int r){
        if(l == r){
            return sz(str)-l;
        }else{
            l = rk[l],r = rk[r];
            if(l>r)swap(l, r);
            l++;
            int lvl = floor(log2(r-l+1)+eps);
            return min(st[lvl][l],st[lvl][r-twop(lvl)+1]);
        }
    }
    
};


int main() {

    
    
    
    
    
    vector<string>v;
    
    string s;cin>>s;
    REP(i,0,sz(s)){
        REP(j,1,6){
            if(i+j<=sz(s)){
                v.PB(s.substr(i,j));
            }
        }
    }
    sort(all(v));
    
    
    int k;in(k);
    complete_unique(v);
    cout<<v[k-1];
    
    
    
    
    
    
    
    
    
    
    
    
    
    return 0;
}


Measured memory_bytes: 1660