import java.util.Scanner;
import java.util.Arrays;
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();
      	String[] strs = new String [N+1];
      	for (int i =0; i<N+1 ; i++){
          	String string = sc.nextLine();
        	strs[i] = string;
        }
        Arrays.sort(strs);
        String str = "";
        for (int i =0; i<N+1 ; i++){
        	str += strs[i];
        }
        System.out.println(str);
   		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