#include <map>
#include <set>
#include <cmath>
#include <ctime>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <cctype>
#include <cstdio>
#include <limits>
#include <string>
#include <vector>
#include <cassert>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iomanip>
#include <numeric>
#include <sstream>
#include <utility>
#include <iostream>
#include <algorithm>
#include <functional>

#define stop exit(0)
#define nc -1
#define eps 1e-10
#define inf 1000000000
#define mod 1000000007
#define mp make_pair

#define fill(array,value) memset(array,value,sizeof(array))
#define f(i,beg,end) for(int i=beg; i<=end; i++)
#define F(i,beg,end) for(int i=beg; i>=end; i--)
#define Max(a,b) ( (a>b)?a:b )
#define Min(a,b) ( (a<b)?a:b )
#define pi 3.1415926535897932384626433832
#define iread(var) scanf("%d",&var)
#define dread(var) scanf("%f",&var)
#define lread(var) scanf("%lld",&var)
#define input(name) freopen(name,"r",stdin)
#define output(name) freopen(name,"w",stdout)
typedef unsigned long long ull;
typedef unsigned int ui;
typedef long double ld;
typedef long long ll;

using namespace std;

int n,m;
string s;

void init()
{
    cin >> n >> m;
    cin.ignore();

    cin >> s;
    sort(s.begin(),s.end());

    int cnt[26];
    fill(cnt,0);

    f(i,0,n-1)
        cnt[s[i]-'a']++;

    string ans;
    f(i,0,25)
        if (cnt[i] && ans.size() < m)
        {
            ans.push_back((char)(i+'a'));
            cnt[i]--;
        }

    if (ans.size() < m)
    {
        f(i,0,25)
            while (cnt[i] && ans.size() < m)
            {
                cnt[i]--;
                ans.push_back((char)(i+'a'));
            }
    }

    sort(ans.begin(),ans.end());

    cout << ans << endl;
}

void solve()
{

}

int main()
{
//	input("test.txt");
    input("letters.in");
    output("letters.out");

	int numberOfTests = 1;
//	cin >> numberOfTests;

	f(i,1,numberOfTests)
	{
		init();
		solve();
	}

	return 0;
}