import java.util.*;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Arrays;
import java.util.Collections;
import java.lang.*;
import java.io.*;

public class Main {
	public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        //ArrayList<Integer> list;
        int n = sc.nextInt();
        int l = sc.nextInt();
        List<String> list = new ArrayList<>();

        for(int i = 0; i < n; i++){
            list.add(sc.next());
        }
        Collections.sort(list);
        //System.out.println(list);
        System.out.println(String.join("", list));

	}
}
/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