import java.util.*;

public class Main{
  public static void main(String[] arg){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
		int M = sc.nextInt();
		String ans = "";
		List<String> list = new ArrayList<>();
		for(int i = 0; i < N; i++){
			list.add(sc.next());
		}
		Collections.sort(list);
		for(int j = 0; j < N; j++){
			ans += list.get(j);
		}
		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