import java.util.*;
import java.util.Collections;
import java.util.ArrayList;

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

        int N = sc.nextInt();
        int L = sc.nextInt();
        String ans = "";
        ArrayList<String> list = new ArrayList<String>();

        for(int i=0; i<N; i++){
            list.add(sc.next());
        }

        Collections.sort(list);

        for(int i=0; i<N; i++){
            ans += list.get(i);
        }
        System.out.println(ans);
    }
}
/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