import java.util.*;
import java.util.Collections;
import java.util.ArrayList;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        int N = sc.nextInt();
        int L = sc.nextInt();
        String ans = "";
        ArrayList<String> list = new ArrayList<String>();

        for(int i=0; i<N; i++){
            list.add(sc.next());
        }

        Collections.sort(list);

        for(int i=0; i<N; i++){
            ans += list.get(i);
        }
        System.out.println(ans);
    }
} import java.util.*; util.* java import java.util.Collections; Collections java.util import java.util.ArrayList; ArrayList 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 ans = "";
        ArrayList<String> list = new ArrayList<String>();

        for(int i=0; i<N; i++){
            list.add(sc.next());
        }

        Collections.sort(list);

        for(int i=0; i<N; i++){
            ans += list.get(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 ans = "";
        ArrayList<String> list = new ArrayList<String>();

        for(int i=0; i<N; i++){
            list.add(sc.next());
        }

        Collections.sort(list);

        for(int i=0; i<N; i++){
            ans += list.get(i);
        }
        System.out.println(ans);
    } main {
        Scanner sc = new Scanner(System.in);

        int N = sc.nextInt();
        int L = sc.nextInt();
        String ans = "";
        ArrayList<String> list = new ArrayList<String>();

        for(int i=0; i<N; i++){
            list.add(sc.next());
        }

        Collections.sort(list);

        for(int i=0; i<N; i++){
            ans += list.get(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 ans = ""; ans "" 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 for(int i=0; i<N; i++){
            ans += list.get(i);
        } int i=0; int i=0; i 0 i<N i N i++ i++ i {
            ans += list.get(i);
        } {
            ans += list.get(i);
        } ans += list.get(i) ans list.get(i) list.get list 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 ans = "";
        ArrayList<String> list = new ArrayList<String>();

        for(int i=0; i<N; i++){
            list.add(sc.next());
        }

        Collections.sort(list);

        for(int i=0; i<N; i++){
            ans += list.get(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 ans = "";
        ArrayList<String> list = new ArrayList<String>();

        for(int i=0; i<N; i++){
            list.add(sc.next());
        }

        Collections.sort(list);

        for(int i=0; i<N; i++){
            ans += list.get(i);
        }
        System.out.println(ans);
    }
} Main
import java.util.*;
import java.util.Collections;
import java.util.ArrayList;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        int N = sc.nextInt();
        int L = sc.nextInt();
        String ans = "";
        ArrayList<String> list = new ArrayList<String>();

        for(int i=0; i<N; i++){
            list.add(sc.next());
        }

        Collections.sort(list);

        for(int i=0; i<N; i++){
            ans += list.get(i);
        }
        System.out.println(ans);
    }
}