// bzoj3998

#include<algorithm>

#include<iostream>

#include<cstdlib>

#include<cstring>

#include<vector>

#include<cstdio>

#include<cmath>

#include<set>

#define LL long long

#define inf 1<<30

#define Pi acos(-1.0)

#define free(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);

using namespace std;

const int maxn=500010;

int n,T,K;

char s[maxn],ans[maxn];

namespace SAM {

	int last,Dargen,sz,n;

	int len[maxn<<1],ch[maxn<<1][26],par[maxn<<1];

	int b[maxn],id[maxn<<1],sum[maxn<<1],r[maxn<<1];

	void Extend(int c) {

		int np=++sz,p=last;last=np;

		len[np]=len[p]+1;

		for (;p && !ch[p][c];p=par[p]) ch[p][c]=np;

		if (!p) par[np]=Dargen;

		else {

			int q=ch[p][c];

			if (len[q]==len[p]+1) par[np]=q;

			else {

				int nq=++sz;len[nq]=len[p]+1;

				memcpy(ch[nq],ch[q],sizeof(ch[q]));

				par[nq]=par[q];

				par[np]=par[q]=nq;

				for (;p && ch[p][c]==q;p=par[p]) ch[p][c]=nq;

			}

		}

	}

	void build() {

		last=Dargen=sz=1;

		n=strlen(s+1);

		for (int i=1;i<=n;i++) Extend(s[i]-'a');

	}

	void pre() {

		for (int i=1;i<=sz;i++) b[len[i]]++;

		for (int i=1;i<=n;i++) b[i]+=b[i-1];

		for (int i=1;i<=sz;i++) id[b[len[i]]--]=i;

		if (!T) {

			for (int i=1;i<=sz;i++) r[i]=1;

			for (int S=0,i=sz;i>=1;i--,S=0) {

				for (int j=0;j<26;j++) if (ch[id[i]][j]) S+=sum[ch[id[i]][j]];

				sum[id[i]]=S+1;

			}

		}

		else {

			for (int p=Dargen,i=1;i<=n;i++) p=ch[p][s[i]-'a'],r[p]++;

			for (int i=sz;i>=1;i--) r[par[id[i]]]+=r[id[i]];

			for (int S=0,i=sz;i>=1;i--,S=0) {

				for (int j=0;j<26;j++) if (ch[id[i]][j]) S+=sum[ch[id[i]][j]];

				sum[id[i]]=S+r[id[i]];

			}

		}

	}

	void query() {

		int tot=0,p=Dargen;

		while (K>0) {

			for (int i=0;i<26;i++) if (ch[p][i]) {

					if (sum[ch[p][i]]>=K) {

						p=ch[p][i];K-=r[p];

						ans[++tot]=i+'a';

						break;

					}

					else K-=sum[ch[p][i]];

				}

		}

		ans[++tot]='\0';

	}

}

using namespace SAM;

int main() {

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

	scanf("%d%d",&T,&K);

	build();

	pre();

	query();

	puts(ans+1);

	return 0;

}