import java.util.*;

public class Main {

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

	}
	
	static void readArray(int[] arr,Scanner sc) { 
		for(int i=0;i<arr.length;i++) {
			arr[i]=sc.nextInt();
		}
	}
	
	
	

}
/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