	
	import java.math.BigInteger;
import java.util.PriorityQueue;
import java.util.Scanner;
	
	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());
			}
			
		}
	}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz