import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		int n = in.nextInt();
		int l = in.nextInt();
		List<String> arrayList = new ArrayList<>();
		in.nextLine();
		for(int i = 0; i < n;i++) {
			arrayList.add(in.nextLine());
		}
		in.close();
		Collections.sort(arrayList);
		for(String e : arrayList) {
			System.out.print(e);
		}

	}
}
/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