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

public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
            Scanner sc = new Scanner(System.in);
            
            int n =sc.nextInt();
            int l =  sc.nextInt();
            String [] str = new String[n];
            
           

            int i=0;
            while (i<n)
            {
                str[i] = sc.next();
                i++;
            }
            Arrays.sort(str);
          for(i=0;i<str.length;i++)
          {
              //System.out.println(str[i]);
          }
          String str2="";
          for(i=0;i<str.length;i++)
          {
              str2 = str2 + str[i];
          }
         
          System.out.println(str2);
    }

}

/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