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

public class Main {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < N; i++) {
      list.add(sc.next());
    }
    Collections.sort(list);
    StringBuilder A = new StringBuilder();
    for (int i = 0; i < N; i++) {
      A.append(list.get(i));
    }
    System.out.println(A);
  }
} import java.util.Scanner; Scanner java.util import java.util.ArrayList; ArrayList java.util import java.util.Collections; Collections 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();
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < N; i++) {
      list.add(sc.next());
    }
    Collections.sort(list);
    StringBuilder A = new StringBuilder();
    for (int i = 0; i < N; i++) {
      A.append(list.get(i));
    }
    System.out.println(A);
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < N; i++) {
      list.add(sc.next());
    }
    Collections.sort(list);
    StringBuilder A = new StringBuilder();
    for (int i = 0; i < N; i++) {
      A.append(list.get(i));
    }
    System.out.println(A);
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < N; i++) {
      list.add(sc.next());
    }
    Collections.sort(list);
    StringBuilder A = new StringBuilder();
    for (int i = 0; i < N; i++) {
      A.append(list.get(i));
    }
    System.out.println(A);
  } 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 ArrayList<String> list = new ArrayList<String>(); list new ArrayList<String>() for (int i = 0; i < N; i++) {
      list.add(sc.next());
    } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
      list.add(sc.next());
    } {
      list.add(sc.next());
    } list.add(sc.next()) list.add list sc.next() sc.next sc Collections.sort(list) Collections.sort Collections list StringBuilder A = new StringBuilder(); A new StringBuilder() for (int i = 0; i < N; i++) {
      A.append(list.get(i));
    } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
      A.append(list.get(i));
    } {
      A.append(list.get(i));
    } A.append(list.get(i)) A.append A list.get(i) list.get list i System.out.println(A) System.out.println System.out System System.out A 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();
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < N; i++) {
      list.add(sc.next());
    }
    Collections.sort(list);
    StringBuilder A = new StringBuilder();
    for (int i = 0; i < N; i++) {
      A.append(list.get(i));
    }
    System.out.println(A);
  }
} public class Main {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < N; i++) {
      list.add(sc.next());
    }
    Collections.sort(list);
    StringBuilder A = new StringBuilder();
    for (int i = 0; i < N; i++) {
      A.append(list.get(i));
    }
    System.out.println(A);
  }
} Main
import java.util.Scanner;
import java.util.ArrayList;
import java.util.Collections;

public class Main {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < N; i++) {
      list.add(sc.next());
    }
    Collections.sort(list);
    StringBuilder A = new StringBuilder();
    for (int i = 0; i < N; i++) {
      A.append(list.get(i));
    }
    System.out.println(A);
  }
}
