import java.util.Arrays;
import java.util.Scanner;

public class Main {

  public static void main(String[] args) {

    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);
    String ans = "";
    
    for (int i = 0; i < n; i++) {
      ans += s[i];
    }
    
    System.out.println(ans);

  }
} import java.util.Arrays; Arrays java.util import java.util.Scanner; Scanner 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[] s = new String[n];

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

    Arrays.sort(s);
    String ans = "";
    
    for (int i = 0; i < n; i++) {
      ans += s[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[] s = new String[n];

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

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

  } 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);
    String ans = "";
    
    for (int i = 0; i < n; i++) {
      ans += s[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[] 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 String ans = ""; ans "" for (int i = 0; i < n; i++) {
      ans += s[i];
    } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
      ans += s[i];
    } {
      ans += s[i];
    } ans += s[i] ans s[i] s 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[] s = new String[n];

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

    Arrays.sort(s);
    String ans = "";
    
    for (int i = 0; i < n; i++) {
      ans += s[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[] s = new String[n];

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

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

  }
} Main
import java.util.Arrays;
import java.util.Scanner;

public class Main {

  public static void main(String[] args) {

    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);
    String ans = "";
    
    for (int i = 0; i < n; i++) {
      ans += s[i];
    }
    
    System.out.println(ans);

  }
}