#include<bits/stdc++.h>

using namespace std;

const int N=1e6+10;

typedef long long ll;

char S[N/2];

char ans[N/2];

int fla,k,top;

struct suffixautomation

{

    int mp[N][30];int fa[N];int ed;int ct;int len[N];int siz[N];

    ll weight[N];

    int z[N],A[N];

    suffixautomation(){ed=ct=1;}

    inline void ins(int c,int pos)

    {

        int p=ed;siz[ed=++ct]=1;len[ed]=pos;//先初始化size和len

        for(;p&&mp[p][c]==0;p=fa[p]){mp[p][c]=ed;}//然后顺着parent树的路径向上找

        if(p==0){fa[ed]=1;return;}int q=mp[p][c];//case1

        if(len[p]+1==len[q]){fa[ed]=q;return;}//case2

        len[++ct]=len[p]+1;//case 3

        for(int i=1;i<=26;i++){mp[ct][i]=mp[q][i];}

        fa[ct]=fa[q];fa[q]=ct;fa[ed]=ct;

        for(int i=p;mp[i][c]==q;i=fa[i]){mp[i][c]=ct;}

    }

    void solve(){

        //sort

        for(int i=1;i<=ct;i++)z[len[i]]++;

        for(int i=1;i<=ct;i++)z[i]+=z[i-1];

        for(int i=1;i<=ct;i++)A[z[len[i]]--]=i;

        for(int i=ct;i>=1;i--){

            if(fla)siz[fa[A[i]]]+=siz[A[i]];

            else siz[fa[A[i]]]=1;

        }

        for(int i=ct;i>=1;i--){

         //   cout<<A[i]<<endl;

            int v=A[i];

            weight[v]=siz[v];

            for(int j=1;j<=26;j++)

                if(mp[v][j])weight[v]+=weight[mp[v][j]];

        }

        weight[1]-=siz[1];

        if(k>weight[1]){

            printf("-1\n");

            return ;

        }

        int now=1;

        while(1){

            for(int i=1;i<=26;i++)

                if(k>weight[mp[now][i]]){

                    k-=weight[mp[now][i]];

                }else{

                    now=mp[now][i];

                    ans[++top]='a'+i-1;

                    break;

                }

            if(k<=siz[now])break;

            else k-=siz[now];

        }

        ans[++top]=0;

        printf("%s\n",ans+1);

    }

}sam;

int main()

{

    scanf("%s",S+1);

    scanf("%d %d",&fla,&k);

    int len=strlen(S+1);

    for(int i=1;i<=len;i++)sam.ins(S[i]-'a'+1,i);

    sam.solve();

    return 0;

}