#include<iostream>

#include<cstdio>

#include<cmath>

#include<cstdlib>

#include<cstring>

#include<algorithm>

using namespace std;

#define ll long long

#define N 1000010

char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}

int gcd(int n,int m){return m==0?n:gcd(m,n%m);}

int read()

{

	int x=0,f=1;char c=getchar();

	while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}

	while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();

	return x*f;

}

int n,op,son[N][26],fail[N],len[N],size[N],id[N],tmp[N],cnt=1,last=1;

ll k,f[N],g[N];

char s[N];

void ins(int c)

{

	int x=++cnt,p=last;last=x;len[x]=len[p]+1;size[x]=1;

	while (!son[p][c]) son[p][c]=x,p=fail[p];

	if (!p) fail[x]=1;

	else

	{

		int q=son[p][c];

		if (len[p]+1==len[q]) fail[x]=q;

		else

		{

			int y=++cnt;

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

			memcpy(son[y],son[q],sizeof(son[q]));

			fail[y]=fail[q],fail[q]=fail[x]=y;

			while (son[p][c]==q) son[p][c]=y,p=fail[p];

		}

	}

}

void dfs(int k,ll x)

{

	x-=g[k];

	if (x==0) return;

	for (int i=0;i<26;i++)

	if (x<=f[son[k][i]])

	{

		putchar(i+'a');

		dfs(son[k][i],x);

		return;

	}

	else x-=f[son[k][i]];

}

int main()

{

#ifndef ONLINE_JUDGE

	freopen("a.in","r",stdin);

	freopen("a.out","w",stdout);

	const char LL[]="%I64d\n";

#else

	const char LL[]="%lld\n";

#endif

	scanf("%s",s+1);n=strlen(s+1);

	cin>>op>>k;

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

	for (int i=1;i<=cnt;i++) tmp[len[i]]++;

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

	for (int i=1;i<=cnt;i++) id[tmp[len[i]]--]=i;

	for (int i=cnt;i>=1;i--) size[fail[id[i]]]+=size[id[i]];

	for (int i=2;i<=cnt;i++) if (op==0) g[i]=1;else g[i]=size[i];

	for (int i=cnt;i>=1;i--)

	{

		int x=id[i];f[x]=g[x];

		for (int j=0;j<26;j++) f[x]+=f[son[x][j]];

	}

	if (f[1]<k) {cout<<-1;return 0;}

	dfs(1,k);

	return 0;

}