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