#include <iostream>
#include <cstdio>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string.h>
#include <queue>
using namespace std;
typedef long long lld;

lld cnt[256];
string ans;
lld n, k;

lld take = 0;

lld MN(lld a, lld b)
{
    if (a < b) return a;
    return b;
}

lld TFO[256];

int main ()
{
    freopen("letters.in", "r", stdin);
    freopen("letters.out", "w", stdout);

    lld i, j, ii, jj, ind, len = 0;
    char a;
    lld diff = 0;

    memset(cnt, 0, sizeof(cnt));
    memset(TFO, 0, sizeof(TFO));

    scanf("%lld %lld", &n, &k);
    for (i=1;i<=n;i++)
    {
        scanf("%c", &a);

        if (a < 'a' || a > 'z')
        {
            i--;
            continue;
        }

        cnt[a] ++;
    }

    for (i=0;i<256 && len < k;i++)
    {
        diff++;

        if (len + cnt[i] <= k)
        {
            len+=cnt[i];

            for (j=1;j<=cnt[i];j++)
            {
                TFO[i] ++;
                //printf("%c", (char)i);
                ans+= (char)i;
            }

            continue;
        }

        else
        {
            for (j=len;j<k;j++)
            {
                TFO[i] ++;
                ans+= (char)i;
            }

            break;
        }
    }

    for (i=ans.size() - 1; i>=0; i--)
    {
        if (TFO[ans[i]] < 2) continue;

        TFO[ans[i]]--;
       // cout<<ans[i]<<" q ima "<<TFO[ans[i]]<<" puti"<<endl;

        for (j='a';j<='z';j++)
        {
            if (cnt[j] < 1) continue;

            if (TFO[j] < 1)
            {
                TFO[j] ++;
                ans[i] = (char)j;
                //cout<<"Poneje "<<(char)j<<" q ima "<<TFO[j]-1<<", slagame neq"<<endl;
                break;
            }
        }

    }

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

    cout<<ans<<endl;
}