#include<stdio.h>
#include<iostream>
#include<queue>
using namespace std;
char s[1000009];
char ts[1000009];
int main()
{
int n,m;
while(scanf("%d%d",&n,&m)!=EOF)
{
int i,j;
scanf("%s",s);
queue<int>qq[10];
int min=9;
if(m==1)
{
for(i=0;i<n;i++)
{
if((s[i]-'0')<min)
min=(s[i]-'0');
}
printf("%d\n",min);
continue;
}
int t=n-m,add=0,qi=0;
for(i=0;i<=t;i++)
{
qq[s[i]-'0'].push(i);
}
for(i=1;i<=9;i++)
{
if(qq[i].size()>=1)
{
ts[add]=i+'0';
qi=qq[i].front();
add++;
break;
}
}
for(i=t+1;i<n;i++)
{
qq[s[i]-'0'].push(i);
for(j=0;j<=9;j++)
{
while(qq[j].size()>0&&qq[j].front()<=qi)
{
qq[j].pop();
}
if(qq[j].size()==0)continue;
ts[add]=j+'0';
qi=qq[j].front();
add++;
break;
}
}
ts[add]=0;
printf("%s\n",ts);
}
}