import java.util.*;

public class Main {
  public static void main(String[] args ) throws Exception {
    Scanner sc = new Scanner(System.in);

    int N = sc.nextInt();
    int L = sc.nextInt();
    
    List<String> ary = new ArrayList();

    for(int i = 0; i < N; i++) {
        String str = sc.next();
        
        if(str.length() == L) {
            ary.add(str);
        }
    }
    
    Collections.sort(ary);
    
    String res = String.join("",ary);

    System.out.println(res);
 
  }
}

/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