
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Scanner;

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */


/**
 *
 * @author mix
 */
 public class Main {

   
    
   
    
    public static void main(String[] args) {
        // TODO code application logic here
        
        
        Scanner s1 = new Scanner(System.in);
        
        int N = s1.nextInt();
        int L = s1.nextInt();
        
        String[] stringArray=new String[N];
        
          for(int i=0;i<N;i++){
            stringArray[i]=s1.next();
            }
          
         
          Arrays.sort(stringArray);
          
          for(String s : stringArray){
          System.out.print(s);
          }
        
          
          System.out.println("");
    }
    
} import java.util.ArrayList; ArrayList java.util import java.util.Arrays; Arrays java.util import java.util.Collections; Collections java.util import java.util.Scanner; Scanner java.util  public class Main {

   
    
   
    
    public static void main(String[] args) {
        // TODO code application logic here
        
        
        Scanner s1 = new Scanner(System.in);
        
        int N = s1.nextInt();
        int L = s1.nextInt();
        
        String[] stringArray=new String[N];
        
          for(int i=0;i<N;i++){
            stringArray[i]=s1.next();
            }
          
         
          Arrays.sort(stringArray);
          
          for(String s : stringArray){
          System.out.print(s);
          }
        
          
          System.out.println("");
    }
    
} Main public static void main(String[] args) {
        // TODO code application logic here
        
        
        Scanner s1 = new Scanner(System.in);
        
        int N = s1.nextInt();
        int L = s1.nextInt();
        
        String[] stringArray=new String[N];
        
          for(int i=0;i<N;i++){
            stringArray[i]=s1.next();
            }
          
         
          Arrays.sort(stringArray);
          
          for(String s : stringArray){
          System.out.print(s);
          }
        
          
          System.out.println("");
    } main {
        // TODO code application logic here
        
        
        Scanner s1 = new Scanner(System.in);
        
        int N = s1.nextInt();
        int L = s1.nextInt();
        
        String[] stringArray=new String[N];
        
          for(int i=0;i<N;i++){
            stringArray[i]=s1.next();
            }
          
         
          Arrays.sort(stringArray);
          
          for(String s : stringArray){
          System.out.print(s);
          }
        
          
          System.out.println("");
    } Scanner s1 = new Scanner(System.in); s1 new Scanner(System.in) int N = s1.nextInt(); N s1.nextInt() s1.nextInt s1 int L = s1.nextInt(); L s1.nextInt() s1.nextInt s1 String[] stringArray=new String[N]; stringArray new String[N] N for(int i=0;i<N;i++){
            stringArray[i]=s1.next();
            } int i=0; int i=0; i 0 i<N i N i++ i++ i {
            stringArray[i]=s1.next();
            } {
            stringArray[i]=s1.next();
            } stringArray[i]=s1.next() stringArray[i] stringArray i s1.next() s1.next s1 Arrays.sort(stringArray) Arrays.sort Arrays stringArray for(String s : stringArray){
          System.out.print(s);
          } String s stringArray {
          System.out.print(s);
          } {
          System.out.print(s);
          } System.out.print(s) System.out.print System.out System System.out s System.out.println("") System.out.println System.out System System.out "" String[] args args  public class Main {

   
    
   
    
    public static void main(String[] args) {
        // TODO code application logic here
        
        
        Scanner s1 = new Scanner(System.in);
        
        int N = s1.nextInt();
        int L = s1.nextInt();
        
        String[] stringArray=new String[N];
        
          for(int i=0;i<N;i++){
            stringArray[i]=s1.next();
            }
          
         
          Arrays.sort(stringArray);
          
          for(String s : stringArray){
          System.out.print(s);
          }
        
          
          System.out.println("");
    }
    
}  public class Main {

   
    
   
    
    public static void main(String[] args) {
        // TODO code application logic here
        
        
        Scanner s1 = new Scanner(System.in);
        
        int N = s1.nextInt();
        int L = s1.nextInt();
        
        String[] stringArray=new String[N];
        
          for(int i=0;i<N;i++){
            stringArray[i]=s1.next();
            }
          
         
          Arrays.sort(stringArray);
          
          for(String s : stringArray){
          System.out.print(s);
          }
        
          
          System.out.println("");
    }
    
} Main

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Scanner;

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */


/**
 *
 * @author mix
 */
 public class Main {

   
    
   
    
    public static void main(String[] args) {
        // TODO code application logic here
        
        
        Scanner s1 = new Scanner(System.in);
        
        int N = s1.nextInt();
        int L = s1.nextInt();
        
        String[] stringArray=new String[N];
        
          for(int i=0;i<N;i++){
            stringArray[i]=s1.next();
            }
          
         
          Arrays.sort(stringArray);
          
          for(String s : stringArray){
          System.out.print(s);
          }
        
          
          System.out.println("");
    }
    
}
