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 A[] =new String[N];
		for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        }
		String ans="";
		
		Arrays.sort(A);
		
		for(int i=0;i<N;i++){
            ans+=A[i];
            
        }
        
        
        System.out.println(ans);
		
		
		
	}
}

/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