import java.util.*;
public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int l = sc.nextInt();
		List<String> s = new ArrayList<String>();
		for(int i=0;i<n;i++) {
			s.add(sc.next());
		}
		Collections.sort(s);
	//	System.out.println(s);
		String ans = "";
		for(int i=0;i<n;i++) {
			ans += s.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