import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int l = sc.nextInt();
        String[] s = new String[n];
        List<String> li = new ArrayList<String>();
        for (int i = 0; i < n; i++) {
            s[i] = sc.next();
            li.add(s[i]);
        }
        Collections.sort(li);
        for (int i = 0; i < n; i++) {
            System.out.print(li.get(i));
        }
        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