import java.util.*;
import java.io.*;

public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int l = scan.nextInt();
    String[] s = new String[n];

    for(int i = 0; i < n; i++){s[i] = scan.next();}

    Arrays.sort(s);
    for(int i = 0; i < n; i++){
      System.out.print(s[i]);
    }
    System.out.println();
  }
} import java.util.*; util.* java import java.io.*; io.* java public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int l = scan.nextInt();
    String[] s = new String[n];

    for(int i = 0; i < n; i++){s[i] = scan.next();}

    Arrays.sort(s);
    for(int i = 0; i < n; i++){
      System.out.print(s[i]);
    }
    System.out.println();
  }
} Main public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int l = scan.nextInt();
    String[] s = new String[n];

    for(int i = 0; i < n; i++){s[i] = scan.next();}

    Arrays.sort(s);
    for(int i = 0; i < n; i++){
      System.out.print(s[i]);
    }
    System.out.println();
  } main {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int l = scan.nextInt();
    String[] s = new String[n];

    for(int i = 0; i < n; i++){s[i] = scan.next();}

    Arrays.sort(s);
    for(int i = 0; i < n; i++){
      System.out.print(s[i]);
    }
    System.out.println();
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int n = scan.nextInt(); n scan.nextInt() scan.nextInt scan int l = scan.nextInt(); l scan.nextInt() scan.nextInt scan String[] s = new String[n]; s new String[n] n for(int i = 0; i < n; i++){s[i] = scan.next();} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {s[i] = scan.next();} {s[i] = scan.next();} s[i] = scan.next() s[i] s i scan.next() scan.next scan Arrays.sort(s) Arrays.sort Arrays s for(int i = 0; i < n; i++){
      System.out.print(s[i]);
    } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
      System.out.print(s[i]);
    } {
      System.out.print(s[i]);
    } System.out.print(s[i]) System.out.print System.out System System.out s[i] s i System.out.println() System.out.println System.out System System.out String[] args args public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int l = scan.nextInt();
    String[] s = new String[n];

    for(int i = 0; i < n; i++){s[i] = scan.next();}

    Arrays.sort(s);
    for(int i = 0; i < n; i++){
      System.out.print(s[i]);
    }
    System.out.println();
  }
} public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int l = scan.nextInt();
    String[] s = new String[n];

    for(int i = 0; i < n; i++){s[i] = scan.next();}

    Arrays.sort(s);
    for(int i = 0; i < n; i++){
      System.out.print(s[i]);
    }
    System.out.println();
  }
} Main
import java.util.*;
import java.io.*;

public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int l = scan.nextInt();
    String[] s = new String[n];

    for(int i = 0; i < n; i++){s[i] = scan.next();}

    Arrays.sort(s);
    for(int i = 0; i < n; i++){
      System.out.print(s[i]);
    }
    System.out.println();
  }
}
