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

public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        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();
        }
        Arrays.sort(s);
        for (int i = 0 ; i < n ; i++) {
            System.out.print(s[i]);
            if (i == n - 1) {
                System.out.print("\n");
            }
        }
    }

}

/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