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

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int num = sc.nextInt();
		int length = sc.nextInt();
		String[] str = new String[num];
		for (int i = 0; i < num; i++) {
			str[i] = sc.next();
		}

		Arrays.sort(str);
		StringBuffer res = new StringBuffer();
		for (String s : str) {
			res.append(s);
		}
		System.out.println(res);

	}
}

/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