import java.util.Scanner;
import java.util.ArrayList;
import java.util.Collections;

public class Main {
	public static void main(String[] args){
		 Scanner sc = new Scanner(System.in);
		 int N=sc.nextInt();
		 int L=sc.nextInt();
		 ArrayList<String> S=new ArrayList<>();
		 for(int i=0;i<N;i++) {
			 S.add(sc.next());
		 }
		 Collections.sort(S);
		 
		 for(int i=0;i<N;i++)
			 System.out.print(S.get(i));
	     sc.close();
	 }
}

/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