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++ ) {
	    String S = sc.next();
	    list.add(S);
	}
	Collections.sort(list);
	StringBuilder sb = new StringBuilder();
	for ( int i = 0; i < list.size(); i++ ) {
	    sb.append(list.get(i));
	}
	String s = sb.toString();
	System.out.println(s);
    }
} 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++ ) {
	    String S = sc.next();
	    list.add(S);
	}
	Collections.sort(list);
	StringBuilder sb = new StringBuilder();
	for ( int i = 0; i < list.size(); i++ ) {
	    sb.append(list.get(i));
	}
	String s = sb.toString();
	System.out.println(s);
    }
} 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++ ) {
	    String S = sc.next();
	    list.add(S);
	}
	Collections.sort(list);
	StringBuilder sb = new StringBuilder();
	for ( int i = 0; i < list.size(); i++ ) {
	    sb.append(list.get(i));
	}
	String s = sb.toString();
	System.out.println(s);
    } 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++ ) {
	    String S = sc.next();
	    list.add(S);
	}
	Collections.sort(list);
	StringBuilder sb = new StringBuilder();
	for ( int i = 0; i < list.size(); i++ ) {
	    sb.append(list.get(i));
	}
	String s = sb.toString();
	System.out.println(s);
    } 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++ ) {
	    String S = sc.next();
	    list.add(S);
	} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
	    String S = sc.next();
	    list.add(S);
	} {
	    String S = sc.next();
	    list.add(S);
	} String S = sc.next(); S sc.next() sc.next sc list.add(S) list.add list S Collections.sort(list) Collections.sort Collections list StringBuilder sb = new StringBuilder(); sb new StringBuilder() for ( int i = 0; i < list.size(); i++ ) {
	    sb.append(list.get(i));
	} int i = 0; int i = 0; i 0 i < list.size() i list.size() list.size list i++ i++ i {
	    sb.append(list.get(i));
	} {
	    sb.append(list.get(i));
	} sb.append(list.get(i)) sb.append sb list.get(i) list.get list i String s = sb.toString(); s sb.toString() sb.toString sb System.out.println(s) System.out.println System.out System System.out s 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++ ) {
	    String S = sc.next();
	    list.add(S);
	}
	Collections.sort(list);
	StringBuilder sb = new StringBuilder();
	for ( int i = 0; i < list.size(); i++ ) {
	    sb.append(list.get(i));
	}
	String s = sb.toString();
	System.out.println(s);
    }
} 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++ ) {
	    String S = sc.next();
	    list.add(S);
	}
	Collections.sort(list);
	StringBuilder sb = new StringBuilder();
	for ( int i = 0; i < list.size(); i++ ) {
	    sb.append(list.get(i));
	}
	String s = sb.toString();
	System.out.println(s);
    }
} 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++ ) {
	    String S = sc.next();
	    list.add(S);
	}
	Collections.sort(list);
	StringBuilder sb = new StringBuilder();
	for ( int i = 0; i < list.size(); i++ ) {
	    sb.append(list.get(i));
	}
	String s = sb.toString();
	System.out.println(s);
    }
}
