import java.util.*;
public class Main {
	public static void main(String[] args) {
		Scanner in=new Scanner (System.in);
		int N=in.nextInt();
		int L=in.nextInt();
		String[] arr=new String[N];
		for (int i = 0; i < N; i++) {
			arr[i]=in.next();
		}
		Arrays.sort(arr);
		String a="";
		for (int i = 0; i < arr.length; i++) {
			a=a+arr[i];
		}
		System.out.println(a);
	}	
}
/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