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 {
		BufferedReader input =new BufferedReader (new InputStreamReader (System.in));
		String str = input.readLine( );
		String[] nl = str.split(" ");
		int n = Integer.parseInt(nl[0]);
		String[] list = new String[n];

		for(int i=0; i<n; i++){
			list[i] = input.readLine( );
		}
		Arrays.sort(list);
		String answer = "";
		for(int i=0; i<n; i++){
			answer = answer + list[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