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(); //N個
		int L = sc.nextInt(); //長さL
		
		String[] stvals = new String[N];
		for(int i=0;i<N;i++) {
			stvals[i] = sc.next();
		}
		Arrays.sort(stvals);
		for(String s:stvals) {
			System.out.print(s);
		}
		System.out.println();
		
		sc.close();
	}	

}
/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