import java.io.*;
import java.util.Arrays;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] inp = br.readLine().split(" ");
		String input = null;
		int n = Integer.parseInt(inp[0]);
		int l = Integer.parseInt(inp[1]);
		String[] in = new String[n];
		for (int i = 0; i < n; i++) {
			in[i] = br.readLine();
		}
		Arrays.sort(in);
		for (int i = 0; i < n; i++) {
			System.out.print(in[i]);
		}
		System.out.println();
	}

}
/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