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();
    List<String> array = new ArrayList<String>();
        
    for(int i=0;i<N;i++){
      String tmp = sc.next();
      array.add(tmp);
    }
    
    Collections.sort(array);
    
    String output = "";
    
    for(int i=0;i<N;i++){
      output += array.get(i);
    }
    
    System.out.println(output);
  }
}

      
/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