import java.util.*;


class Main{
    public static void main(String args[]){
        Scanner scan = new Scanner(System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        List<String> array = new ArrayList<String>();
        int i = 0;

        while(i<=a){
          String c = scan.nextLine();
          array.add(c);
          i++;  
        }
        Collections.sort(array);
        i = 0;
        while(i<=a){
            System.out.print(array.get(i));
            i++;  
          }
        scan.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