import java.util.*;
import java.io.*;
public class Main{
	public static void main(String[] args) throws IOException{
    	BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String[] list = reader.readLine().split(" ");
        int t = Integer.parseInt(list[0]);
        String[] l = new String[t];
        for(int i = 0 ;i<t;i++){
            l[i] = reader.readLine();
        }
        Arrays.sort(l);
        for(String p:l){
        	System.out.print(p);
            }
	}}
        
/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