import java.util.*;

class Main{
	public static void main(String args []){
    Scanner scan = new Scanner(System.in);
      int n = scan.nextInt();
      int l = scan.nextInt();
      String [] ss = new String[n];
      int i=0;
      while(i<n)
      {
        ss[i]=scan.next();
        i++;
      }
      Arrays.sort(ss);
      String res = "";
      for(String s:ss){
      	res+=s;
      }
      
      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