import java.util.*;
 
public class Main {
	
    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int L = sc.nextInt();
    	List<String> list = new ArrayList<>();
    	for (int i = 0; i < N; i++) {
    		list.add(sc.next());
    	}
    	Collections.sort(list);
    	StringBuilder sb = new StringBuilder();
    	for (String s : list) {
    		sb.append(s);
    	}
    	System.out.println(sb);
    }
}
/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