#define _CRT_SECURE_NO_DEPRECATE
#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <ctime>
#include <queue>
#include <algorithm>
#include <iomanip>
#include <map>
#include <set>
#include <math.h>
#include <stack>
#include <deque>
#include <numeric>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <bitset>

#define all(v) v.begin(),v.end()
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
const ld epsylon = 1e-9;
typedef unsigned int ui;
inline long double get_time(){   
	return (long double)clock()/CLOCKS_PER_SEC;
};
ld start_time,end_time;

int main()
{
	freopen("letters.in","r",stdin);
	freopen("letters.out","w",stdout);
	//start_time = get_time();
	//program
	int n, k;
	string niz;
	int alphabet[255];
	int res[255];
	fill(alphabet, alphabet+255,0);
	fill(res, res+255,0);
	cin >> n >> k >> niz;
	for (int i = 0; i < niz.size(); ++i) {
		alphabet[niz[i]]++;
	}
	int z = 0;
	for (int i = 0; i < 255; ++i) {
		if (alphabet[i]>0) {
			res[i]++;
			z++;
			alphabet[i]--;
			if (z>=k) break;
		}
	}
	for (int i = 0; i < 255; ++i) {
		while (alphabet[i]>0) {
			res[i]++;
			alphabet[i]--;
			z++;
			if (z>=k) break;
		}
		if (z>=k) break;
	}
	string result = "";
	for (int i = 0; i < 255; ++i) {
		while(res[i]>0) {
			result.push_back((char)i);
			res[i]--;
		}
	}
	cout << result;
	//end program
	//end_time=get_time()-start_time;
	return 0;
}