import java.util.*;
public class Main {

	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt(), l=sc.nextInt();
		sc.nextLine();
		String res="";
		String[] str=new String[n];
		for(int i=0; i<n; ++i) str[i]=sc.nextLine(); 
		Collections.sort(Arrays.asList(str));
		for(int i=0; i<n; ++i) res+=str[i];
		System.out.println(res);
		sc.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