import java.util.Arrays;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        int l = sc.nextInt();

        String[] s = new String[n];

        for (int i = 0; i < n; i++) {
            s[i] = sc.next();
        }

        sc.close();

        Arrays.sort(s);

        String ans = "";

        for (int i = 0; i < n; i++) {
            ans = ans + s[i];
        }

        System.out.println(ans);

    }
}
/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