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

public class Main {

	public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int l = scan.nextInt();
		List<String> s = new ArrayList<String>();

		for (int i = 0; i < n; i++) {
			s.add(scan.next());
		}
		s.sort(Comparator.naturalOrder());
		System.out.println(String.join("", s));
	}

}

/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