import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
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();
		ArrayList<String> list = new ArrayList<>();
		sc.nextLine();
		for(int i = 0; i < n; i++) {
			list.add(sc.nextLine());
		}
		Collections.sort(list, Comparator.naturalOrder());
		StringBuilder sb = new StringBuilder();
		for(String str: list) {
			sb.append(str);
		}
		System.out.println(sb.toString());
		
	}
}
/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