import java.util.*;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);		
		int N = sc.nextInt();		
		int L = sc.nextInt();
		String s[]= new String[N];

		for(int i=0; i<N; i++){
		 s[i] = sc.next();
		}		
		Arrays.sort(s);
		for (String str : s) System.out.print(str);
        System.out.println();
		
	}	
}
/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