86分超时求助！！

#include<bits/stdc++.h>
using namespace std;
string s,ans="99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999";
int k,m,l,o=0,f=0;
void dfs(int x,int p,string a)
{
//	cout<<a<<'\n';
    if(p>m)return;
	if(p==m)
	{
		if(a!="")
		    ans=min(ans,a);
		return;
	}
	if(x>l){
		return;
	}
	dfs(x+1,p+1,a+s[x-1]);
	dfs(x+1,p,a);
}
int main()
{
	cin>>s>>k;
	l=s.size();
	m=l-k;
	dfs(1,0,"");
	for(int i=0;i<ans.size();i++)
	{
		f=f*10+ans[i]-'0';
	}
	cout<<f;
}