import java.util.*;
import java.util.Arrays;


public class Main{
  
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    String [] x = new String[N];
    for(int i=0;i<N;i++){
      x[i] = sc.next();
      
    }
    Arrays.sort(x);
    String ans = "";
    for(int i=0;i<N;i++){
      ans += x[i];
    }

    
    System.out.println(ans);
    
    
  }
}
                          
                         
                    
    
    
    
    
    
     import java.util.*; util.* java import java.util.Arrays; Arrays java.util public class Main{
  
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    String [] x = new String[N];
    for(int i=0;i<N;i++){
      x[i] = sc.next();
      
    }
    Arrays.sort(x);
    String ans = "";
    for(int i=0;i<N;i++){
      ans += x[i];
    }

    
    System.out.println(ans);
    
    
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    String [] x = new String[N];
    for(int i=0;i<N;i++){
      x[i] = sc.next();
      
    }
    Arrays.sort(x);
    String ans = "";
    for(int i=0;i<N;i++){
      ans += x[i];
    }

    
    System.out.println(ans);
    
    
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    String [] x = new String[N];
    for(int i=0;i<N;i++){
      x[i] = sc.next();
      
    }
    Arrays.sort(x);
    String ans = "";
    for(int i=0;i<N;i++){
      ans += x[i];
    }

    
    System.out.println(ans);
    
    
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int L = sc.nextInt(); L sc.nextInt() sc.nextInt sc String [] x = new String[N]; x new String[N] N for(int i=0;i<N;i++){
      x[i] = sc.next();
      
    } int i=0; int i=0; i 0 i<N i N i++ i++ i {
      x[i] = sc.next();
      
    } {
      x[i] = sc.next();
      
    } x[i] = sc.next() x[i] x i sc.next() sc.next sc Arrays.sort(x) Arrays.sort Arrays x String ans = ""; ans "" for(int i=0;i<N;i++){
      ans += x[i];
    } int i=0; int i=0; i 0 i<N i N i++ i++ i {
      ans += x[i];
    } {
      ans += x[i];
    } ans += x[i] ans x[i] x i System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main{
  
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    String [] x = new String[N];
    for(int i=0;i<N;i++){
      x[i] = sc.next();
      
    }
    Arrays.sort(x);
    String ans = "";
    for(int i=0;i<N;i++){
      ans += x[i];
    }

    
    System.out.println(ans);
    
    
  }
} public class Main{
  
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    String [] x = new String[N];
    for(int i=0;i<N;i++){
      x[i] = sc.next();
      
    }
    Arrays.sort(x);
    String ans = "";
    for(int i=0;i<N;i++){
      ans += x[i];
    }

    
    System.out.println(ans);
    
    
  }
} Main
import java.util.*;
import java.util.Arrays;


public class Main{
  
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    String [] x = new String[N];
    for(int i=0;i<N;i++){
      x[i] = sc.next();
      
    }
    Arrays.sort(x);
    String ans = "";
    for(int i=0;i<N;i++){
      ans += x[i];
    }

    
    System.out.println(ans);
    
    
  }
}
                          
                         
                    
    
    
    
    
    
    
