import java.util.Arrays;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int l = sc.nextInt();

		String[] s = new String[n];
		for (int i = 0; i < n; i++) {
			s[i] = sc.next();
		}

		Arrays.sort(s);

		StringBuilder ans = new StringBuilder(n * l);

		for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		}

		System.out.println(ans);

		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