import java.util.*;
public class Main {
	public static void main(String arg[]) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int l = sc.nextInt();
		
		List<String> lst = new ArrayList<>();
		for(int i = 0; i < n; i++) {
			String s = sc.next();
			lst.add(s);
		}
		
		Collections.sort(lst);

		StringBuffer sb = new StringBuffer();
		for(int i = 0; i < n; i++) {
			sb.append(lst.get(i));
		}
		
		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