import 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();
      
        List<String> list = new ArrayList<String>();
      
        for (int i = 0; i < N; i++){
          list.add(sc.next());
        }
      
        // 並び替え
        Collections.sort(list);
      
        // 回答提出用
        StringBuilder answer = new StringBuilder();
      
        for (int i = 0; i < N; i++){
          answer.append(list.get(i));
        }
       
        System.out.println(answer);

    }
} import java.util.*; util.* java public class Main {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
      
        int N = sc.nextInt();
        int L = sc.nextInt();
      
        List<String> list = new ArrayList<String>();
      
        for (int i = 0; i < N; i++){
          list.add(sc.next());
        }
      
        // 並び替え
        Collections.sort(list);
      
        // 回答提出用
        StringBuilder answer = new StringBuilder();
      
        for (int i = 0; i < N; i++){
          answer.append(list.get(i));
        }
       
        System.out.println(answer);

    }
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
      
        int N = sc.nextInt();
        int L = sc.nextInt();
      
        List<String> list = new ArrayList<String>();
      
        for (int i = 0; i < N; i++){
          list.add(sc.next());
        }
      
        // 並び替え
        Collections.sort(list);
      
        // 回答提出用
        StringBuilder answer = new StringBuilder();
      
        for (int i = 0; i < N; i++){
          answer.append(list.get(i));
        }
       
        System.out.println(answer);

    } main {
		Scanner sc = new Scanner(System.in);
      
        int N = sc.nextInt();
        int L = sc.nextInt();
      
        List<String> list = new ArrayList<String>();
      
        for (int i = 0; i < N; i++){
          list.add(sc.next());
        }
      
        // 並び替え
        Collections.sort(list);
      
        // 回答提出用
        StringBuilder answer = new StringBuilder();
      
        for (int i = 0; i < N; i++){
          answer.append(list.get(i));
        }
       
        System.out.println(answer);

    } 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 List<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 answer = new StringBuilder(); answer new StringBuilder() for (int i = 0; i < N; i++){
          answer.append(list.get(i));
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
          answer.append(list.get(i));
        } {
          answer.append(list.get(i));
        } answer.append(list.get(i)) answer.append answer list.get(i) list.get list i System.out.println(answer) System.out.println System.out System System.out answer 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();
      
        List<String> list = new ArrayList<String>();
      
        for (int i = 0; i < N; i++){
          list.add(sc.next());
        }
      
        // 並び替え
        Collections.sort(list);
      
        // 回答提出用
        StringBuilder answer = new StringBuilder();
      
        for (int i = 0; i < N; i++){
          answer.append(list.get(i));
        }
       
        System.out.println(answer);

    }
} public class Main {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
      
        int N = sc.nextInt();
        int L = sc.nextInt();
      
        List<String> list = new ArrayList<String>();
      
        for (int i = 0; i < N; i++){
          list.add(sc.next());
        }
      
        // 並び替え
        Collections.sort(list);
      
        // 回答提出用
        StringBuilder answer = new StringBuilder();
      
        for (int i = 0; i < N; i++){
          answer.append(list.get(i));
        }
       
        System.out.println(answer);

    }
} Main
import 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();
      
        List<String> list = new ArrayList<String>();
      
        for (int i = 0; i < N; i++){
          list.add(sc.next());
        }
      
        // 並び替え
        Collections.sort(list);
      
        // 回答提出用
        StringBuilder answer = new StringBuilder();
      
        for (int i = 0; i < N; i++){
          answer.append(list.get(i));
        }
       
        System.out.println(answer);

    }
}