#include<cstdio>

#include<iostream>

#include<cstring>

#include<algorithm>

#include<string>

#define LL long long

using namespace std;

const int M=1e7+;

int n,k,cnt;

char s[M],q[M];

int main()

{

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

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

    scanf("%s",s+);

    n=strlen(s+); scanf("%d",&k);

    int v=n-k;

    q[++cnt]=s[];

    for(int i=;i<=n;i++){

        while(cnt&&q[cnt]>s[i]&&v) cnt--,v--;

        q[++cnt]=s[i];

    }

    for(int i=;i<=k;i++) printf("%c",q[i]); printf("\n");

    return ;

}

#include<cstdio>

#include<cstring>

const int M=;

int n,k,c[M][],deep[M];

char s[M];

void maxs(int& x,int y){if(x<y) x=y;}

int main(){

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

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

    scanf("%s",s+);

    n=strlen(s+);

    scanf("%d",&k);

    for(int i=;i<=n;i++){

        int now=s[i]-'a';

        for(int j=i;j&&!c[j][now];c[j--][now]=i+);

    }

    for(int i=n;i>=;i--){

        for(int j=;j<;j++){

            int v=c[i][j];

            if(v) maxs(deep[i],deep[v]+);

        }

    }

    int now=;

    for(int l=k;l;l--){

        for(int j=;j<;j++){

            int v=c[now][j];

            if(v&&deep[v]>=l-){

                now=v;

                putchar('a'+j);

                break;

            }

        }

    }

    return ;

}