import java.util.Arrays;
import java.util.Collections;
import java.util.Scanner;

class Main {
    public static void main(String[] args) {

        Scanner inpunt = new Scanner(System.in);
        int x ,y ;
        String result="";
        x=inpunt.nextInt();
        y=inpunt.nextInt();
        inpunt.nextLine();
        String array[] = new String[x];
        for(int i=0;i<x;i++){
           array[i]=inpunt.nextLine();
        }

        Arrays.sort(array);

        for(int i=0;i<x;i++){
            result += array[i];
        }
        System.out.println(result);

    }
}
/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