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 sc = new Scanner(System.in);
		try {
			int n = sc.nextInt();
			// L
			sc.nextInt();
			sc.nextLine();

			List<String> list = new ArrayList<String>();
			for (int i = 0; i < n; i++) {
				list.add(sc.nextLine());
			}
			Collections.sort(list);

			for (int i = 0; i < n; i++) {
				System.out.print(list.get(i));
			}
			System.out.println();

		} finally {
			sc.close();
		}
	}
}
/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