import java.util.*;

public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		String[] line = sc.nextLine().split(" ");
		int n = Integer.parseInt(line[0]);
		
		List<String> list = new ArrayList<String>();

		String min = "";
		
		for(int i=0;i<n;i++){
			list.add(sc.nextLine());
		}
		sc.close();
		
		Collections.sort(list);
		for(String col:list) {
			min += col;
		}
		System.out.println(min);
	}
}
/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