import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Scanner;

class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int l = sc.nextInt();
    StringBuilder sb = new StringBuilder();
    List<String> sList = new ArrayList<String>();
    for (int i = 0 ; i < n ; i++){
      sList.add(sc.next());
    }
    sList.sort(Comparator.naturalOrder());
    for (String s : sList){
      sb.append(s);
    }
    System.out.println(sb);
    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