//By SiriusRen#include <bits/stdc++.h>using namespace std;const int N=1000050;int n,cntA[N],cntB[N],A[N],B[N],rk[N],sa[N],tsa[N],ht[N],T,K;long long sum[N],sum1[N];char s[N];void SA(){    for(int i=1;i<=n;i++)cntA[s[i]]++;    for(int i=1;i<=256;i++)cntA[i]+=cntA[i-1];    for(int i=n;i;i--)sa[cntA[s[i]]--]=i;    rk[sa[1]]=1;    for(int i=2;i<=n;i++)rk[sa[i]]=rk[sa[i-1]]+(s[sa[i]]!=s[sa[i-1]]);    for(int l=1;rk[sa[n]]<n;l<<=1){        memset(cntA,0,sizeof(cntA));        memset(cntB,0,sizeof(cntB));        for(int i=1;i<=n;i++)cntA[A[i]=rk[i]]++,cntB[B[i]=(i+l<=n?rk[i+l]:0)]++;        for(int i=1;i<=n;i++)cntA[i]+=cntA[i-1],cntB[i]+=cntB[i-1];        for(int i=n;i;i--)tsa[cntB[B[i]]--]=i;        for(int i=n;i;i--)sa[cntA[A[tsa[i]]]--]=tsa[i];        rk[sa[1]]=1;        for(int i=2;i<=n;i++)rk[sa[i]]=rk[sa[i-1]]+(A[sa[i]]!=A[sa[i-1]]||B[sa[i]]!=B[sa[i-1]]);    }    for(int i=1,j=0;i<=n;i++){        j=j?j-1:0;        while(s[i+j]==s[sa[rk[i]-1]+j])j++;        ht[rk[i]]=j;    }}void print(int l,int r){for(int i=l;i<=r;i++)putchar(s[i]);}bool check(int p){    int tempans=1,l=1,r=n;    while(l<=r){        int mid=(l+r)>>1;        if(sum[mid]>=p)r=mid-1;        else tempans=mid+1,l=mid+1;    }    int hi=p-sum[tempans-1]+ht[tempans],tot=hi+sum1[tempans-1];    if(tot>=K)return 1;    for(int i=tempans+1;i<=n;i++){        hi=min(hi,ht[i]);        if(!hi)break;        tot+=hi;        if(tot>=K)return 1;    }return 0;}void solve(){    for(int i=1,t;i<=n;i++,K-=t){        t=n-sa[i]-ht[i]+1;        if(K<=t){print(sa[i],sa[i]+K-1+ht[i]);return;}    }puts("-1");}signed main(){    scanf("%s%d%d",s+1,&T,&K),n=strlen(s+1),SA();    if(!T)solve();    else{        for(int i=1;i<=n;i++)sum[i]=sum[i-1]+n-ht[i]-sa[i]+1,sum1[i]=sum1[i-1]+n-sa[i]+1;        if(sum1[n]<K){puts("-1");return 0;}        int l=1,r=K+1,ans;        while(l<=r){            int mid=(l+r)>>1;            if(check(mid))ans=mid,r=mid-1;            else l=mid+1;        }K=ans;solve();    }}