
import java.util.*;

class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int l = sc.nextInt();
		
		PriorityQueue<String> list = new PriorityQueue<>();
		for (int i = 0; i < n; i++) {
			list.add(sc.next());
		}
		
		for (int i = 0; i < n; i++) {
			System.out.print(list.poll());
		}
	}
} import java.util.*; util.* java class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int l = sc.nextInt();
		
		PriorityQueue<String> list = new PriorityQueue<>();
		for (int i = 0; i < n; i++) {
			list.add(sc.next());
		}
		
		for (int i = 0; i < n; i++) {
			System.out.print(list.poll());
		}
	}
} Main public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int l = sc.nextInt();
		
		PriorityQueue<String> list = new PriorityQueue<>();
		for (int i = 0; i < n; i++) {
			list.add(sc.next());
		}
		
		for (int i = 0; i < n; i++) {
			System.out.print(list.poll());
		}
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int l = sc.nextInt();
		
		PriorityQueue<String> list = new PriorityQueue<>();
		for (int i = 0; i < n; i++) {
			list.add(sc.next());
		}
		
		for (int i = 0; i < n; i++) {
			System.out.print(list.poll());
		}
	} 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 PriorityQueue<String> list = new PriorityQueue<>(); list new PriorityQueue<>() 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 for (int i = 0; i < n; i++) {
			System.out.print(list.poll());
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			System.out.print(list.poll());
		} {
			System.out.print(list.poll());
		} System.out.print(list.poll()) System.out.print System.out System System.out list.poll() list.poll list String args[] args

import java.util.*;

class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int l = sc.nextInt();
		
		PriorityQueue<String> list = new PriorityQueue<>();
		for (int i = 0; i < n; i++) {
			list.add(sc.next());
		}
		
		for (int i = 0; i < n; i++) {
			System.out.print(list.poll());
		}
	}
}