import java.util.*;

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> list = new ArrayList<>();
      for(int i=0;i<n;i++){
        list.add(sc.next());
      }
      Collections.sort(list);
      String output = "";
      for(String s:list){
        output = output+s;
      }
      System.out.println(output);
    }
}
/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