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<>();
    	for (int i = 0; i < N; i++) {
    		list.add(sc.next());
    	}
    	Collections.sort(list);
    	StringBuilder sb = new StringBuilder();
    	for (String s : list) {
    		sb.append(s);
    	}
    	System.out.println(sb);
    }
} 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<>();
    	for (int i = 0; i < N; i++) {
    		list.add(sc.next());
    	}
    	Collections.sort(list);
    	StringBuilder sb = new StringBuilder();
    	for (String s : list) {
    		sb.append(s);
    	}
    	System.out.println(sb);
    }
} 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<>();
    	for (int i = 0; i < N; i++) {
    		list.add(sc.next());
    	}
    	Collections.sort(list);
    	StringBuilder sb = new StringBuilder();
    	for (String s : list) {
    		sb.append(s);
    	}
    	System.out.println(sb);
    } main {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int L = sc.nextInt();
    	List<String> list = new ArrayList<>();
    	for (int i = 0; i < N; i++) {
    		list.add(sc.next());
    	}
    	Collections.sort(list);
    	StringBuilder sb = new StringBuilder();
    	for (String s : list) {
    		sb.append(s);
    	}
    	System.out.println(sb);
    } 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<>(); list new ArrayList<>() 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 sb = new StringBuilder(); sb new StringBuilder() for (String s : list) {
    		sb.append(s);
    	} String s list {
    		sb.append(s);
    	} {
    		sb.append(s);
    	} sb.append(s) sb.append sb s System.out.println(sb) System.out.println System.out System System.out sb 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<>();
    	for (int i = 0; i < N; i++) {
    		list.add(sc.next());
    	}
    	Collections.sort(list);
    	StringBuilder sb = new StringBuilder();
    	for (String s : list) {
    		sb.append(s);
    	}
    	System.out.println(sb);
    }
} 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<>();
    	for (int i = 0; i < N; i++) {
    		list.add(sc.next());
    	}
    	Collections.sort(list);
    	StringBuilder sb = new StringBuilder();
    	for (String s : list) {
    		sb.append(s);
    	}
    	System.out.println(sb);
    }
} 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<>();
    	for (int i = 0; i < N; i++) {
    		list.add(sc.next());
    	}
    	Collections.sort(list);
    	StringBuilder sb = new StringBuilder();
    	for (String s : list) {
    		sb.append(s);
    	}
    	System.out.println(sb);
    }
}