import java.util.Arrays;
import java.util.Scanner;
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 = null;
		String [] array = new String [n];
		for (int i =0; i < n; i++) {
			array[i] = sc.next();
		}
		Arrays.sort(array);
		for (int i = 0; i < n; i ++) {
			ans += array[i];
		}
		System.out.println(ans.substring(ans.length()- (n * l), ans.length()));
	}
}

/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