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();
		sc.nextLine();
		String[] s = new String[N];
		for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		}
		sc.close();
		Arrays.sort(s);
		for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		}
		System.out.println("");
	}
}
/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