LG3975

#include<bits/stdc++.h>

using namespace std;

#define gc c=getchar()
#define r(x) read(x)
#define ll long long

template<typename T>
inline void read(T&x){
    x=0;T k=1;char gc;
    while(!isdigit(c)){if(c=='-')k=-1;gc;}
    while(isdigit(c)){x=x*10+c-'0';gc;}x*=k;
}

const int N=1e6+7;

int len[N];
int to[N][26];
int fa[N];
int siz[N];

int tot;

inline int NewNode(int l,int *tr,int su){
    ++tot;
    len[tot]=l;
    if(tr!=NULL)memcpy(to[tot],tr,26<<2);
    fa[tot]=su;
    return tot;
}

inline int insert(int u,int c){
    int t=NewNode(len[u]+1,NULL,0);
    for(;u&&!to[u][c];u=fa[u])to[u][c]=t;
    if(!u){
        fa[t]=1;
        return t;
    }
    int v=to[u][c];
    if(len[v]==len[u]+1){
        fa[t]=v;
        return t;
    }
    int x=NewNode(len[u]+1,to[v],fa[v]);
    fa[t]=fa[v]=x;
    for(;u&&to[u][c]==v;u=fa[u])to[u][c]=x;
    return t;
}

int t,k;

ll sum[N];

int ord[N];
int cnt[N];
char s[N];

int main(){
    scanf("%s",s);
    int n=strlen(s);
    int lst=NewNode(0,NULL,0);
    for(int i=0;i<n;++i){
        siz[lst=insert(lst,s[i]-'a')]=1;
    }
    for(int i=1;i<=tot;++i)cnt[len[i]]++;
    for(int i=1;i<=n;++i)cnt[i]+=cnt[i-1];
    for(int i=1;i<=tot;++i)ord[cnt[len[i]]--]=i;
    r(t),r(k);
    if(t){
        for(int i=tot;i>1;--i){
            int x=ord[i];
            siz[fa[x]]+=siz[x];
        }
        siz[1]=0;
    }
    else {
        for(int i=2;i<=tot;++i){
            siz[i]=1;
        }
    }
    for(int i=tot;i;--i){
        int x=ord[i];
        sum[x]=siz[x];
        for(int j=0;j<26;++j){
            sum[x]+=sum[to[x][j]];
        }
    }
    if(sum[1]<k)puts("-1");
    else {
        int u=1;
        while(k>0){
            for(int i=0;i<26;++i){
                if(sum[to[u][i]]<k)k-=sum[to[u][i]];
                else {
                    u=to[u][i];
                    putchar(i+'a');
                    k-=siz[u];
                    break;
                }
            }
        }
    }
}