import java.io.BufferedReader;
import java.io.IOException;
import java.io.FileWriter;
import java.io.PrintWriter;
import java.nio.charset.StandardCharsets;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.File;
import java.util.*;

public class Main {

    public static StringBuilder output = new StringBuilder();

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int length = in.nextInt();
        String arr[] = new String[n];
        for (int i = 0; i < n; i++) {
            arr[i] = in.next();
        }
        Solver solver = new Solver();
        solver.solve(arr);
        System.out.println(output);
    }

}

class Solver {
    public void solve(String arr[]) {
        String a = "";
        Arrays.sort(arr);
        for (String s : arr) {
            a = a.concat(s);
        }
        Main.output.append(a);

    }
}
/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