import java.util.ArrayList;
import java.util.Collections;
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();
		List<String> inputs = new ArrayList<String>();
		for(int i = 0; i < N; i++) {
			inputs.add(sc.next());
		}
		Collections.sort(inputs);
		String output = String.join("", inputs);
		System.out.println(output);
	}
}
/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