#include<iostream>
#include<algorithm>
#include<fstream>

using namespace std;



int main()
{
    int n, k;
    string str, new_str;
    int br = 0;
    int count[ 30 ];
    bool used[ 30 ];
    fstream fl, fl2;
    
    for ( int i = 0; i < 30; i++ ) count[ i ] = used[ i ] = 0;
    
    fl.open( "letters.in", ios :: in );
    fl >> n;
    fl >> k;
    fl >> str;
    fl.close();
    
    sort( str.begin(), str.end() );
    
    for ( int i = 0; i < n; i++ ) count[ str[ i ] - 'a' ]++;
    for ( int i = 0; i < n; i++ )
    {
        if ( !used[ str[ i ] - 'a' ] )
        {
            new_str += str[ i ];
            used[ str[ i ] - 'a' ] = 1;
            count[ str[ i ] - 'a' ]--;
            br++;
            if ( br == k ) break;
        }
    }
    if ( br < k )
    {
        for ( int i = 0; i < n; i++ )
        {
            if ( count[ str[ i ] - 'a' ] > 0 )
            {
                for ( int j = count[ str[ i ] - 'a' ]; j >= 1; j-- )
                {
                    if ( new_str.size() == k ) break;
                    new_str += str[ i ];
                    count[ str[ i ] - 'a' ]--;
                }
            }
            if ( new_str.size() == k ) break;
        }
    }
    
    sort( new_str.begin(), new_str.end() );
    
    fl2.open( "letters.out", ios :: out );
    fl2 << new_str;
    fl2.close();
    
    return 0;
}