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

public class Main {
public static void main(String[] args) {
		
		Scanner keyboard = new Scanner(System.in); 
		
		int N = keyboard.nextInt();
		int L = keyboard.nextInt();
		
		String[] SI = new String[N];
		
		for(int i = 0; i < N; i ++) {
			SI[i] = keyboard.next();
		}
		
		Arrays.sort(SI);
		
		for(int i = 0; i < N; i ++) {
			System.out.print(SI[i]);
		}
		keyboard.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