import java.util.*;

class Main{
  public static void main(String[] args){
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      int L = sc.nextInt();
      String[] array = new String[N];
      
      for(int i = 0 ; i < N ; i++){
        array[i] = sc.next();
      }

      Arrays.sort(array);

      for (int j = 0 ; j < array.length ; j++){
        System.out.print(array[j]);

      }
      sc.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