import java.util.Scanner;
import java.util.Arrays;
 
public class Main {
    public static void main(String[] args) {
      Scanner scan = new Scanner(System.in);
      int N = scan.nextInt();
      int L = scan.nextInt();
     
      String[] str = new String[N];
      
      for(int i = 0; i < N; i++){  
        str[i] = scan.next();
      }
      
      Arrays.sort(str);
      
      for(String out : str){
        System.out.print(out);
      }
      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