import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;

public class Main {

	public static void main(String[] args) throws IOException {
		// TODO 自動生成されたメソッド・スタブ
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line[]=br.readLine().split(" ");
		int n =Integer.parseInt(line[0]);
		String [] sort =new String[n];

		for (int i = 0; i < sort.length; i++) {
			sort[i] = br.readLine();
		}

		Arrays.sort(sort);

		StringBuilder answer =new StringBuilder();

		for(int i=0;i<sort.length;i++) {
		answer.append(sort[i]);
		}

		System.out.println(answer);
	}

}

/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