#pragma comment(linker, "/STACK:60777216")  

#include <cassert>
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<complex>
#include<vector>
#include<set>
#include<map>
#include<cmath>
#include<queue>
#include<string>
#include<cstdlib>
#include<memory.h>
#include<ctime>
#include<bitset>
#include<fstream>
#include<queue>

using namespace std;


typedef long double ld;

typedef long long ll;
typedef pair<int,int>	pii;
typedef pair<ld,ld>	pdd;
typedef vector<int> vi;
typedef vector<ld> vd;
typedef pair<ll,ll> pl;

#define FOR(i,a,b)		for(int i=(a);i<(b);i++)
#define REP(i,n)		FOR(i,0,n)
#define SORT(v)			sort((v).begin(),(v).end())
#define UN(v)			SORT(v),(v).erase(unique((v).begin(),(v).end()),(v).end())
#define CL(a,b)			memset(a,b,sizeof a)
#define pb				push_back

int n,k;
string s;
int c[33];

int main(){	
#ifdef LocalHost
	freopen("input.txt","r",stdin);
	//freopen("output.txt","w",stdout);
#endif
#ifndef LocalHost
	freopen("letters.in","r",stdin);
	freopen("letters.out","w",stdout);
#endif

	cin>>n>>k>>s;
	sort(s.begin(),s.end());
	REP(i,s.size()) c[s[i]-'a']++;
	string t;
	REP(i,26)if(k&&c[i]){
		k--;
		c[i]--;
		t.pb(char('a'+i));
	}
	REP(i,26)while(k&&c[i]){
		k--;
		c[i]--;
		t.pb(char('a'+i));
	}

	sort(t.begin(),t.end());
	cout<<t<<endl;

#ifdef LocalHost
	printf("TIME: %.3lf\n",ld(clock())/CLOCKS_PER_SEC);
#endif
	return 0;
}