import java.util.Scanner;

public class Main{
	
	public static void main(String... args){
		Scanner scan = new Scanner(System.in);
		String answer = "";
		
		final int num = scan.nextInt();
		int len = scan.nextInt();
		String[] words = new String[num];
		
		for(int i = 0; i < num; i++){
			words[i] = scan.next();
		}
		
		// ソート
		for(int i = 1; i < num; i++){
			String temp = words[i];
			int j = i - 1;
			while(j >= 0 && words[j].compareTo(temp) > 0){
				words[j + 1] = words[j];
				j--;
			}
			words[j + 1] = temp;
		}
		
		// 全単語の結合
		for(int i = 0; i < num; i++){
			answer = answer.concat(words[i]);
		}
		
		System.out.println(answer);
		
	}
}
/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