import java.util.Arrays;
import java.util.Scanner;
import java.util.stream.Collectors;

public class Main {
  public static void main(String[] args) {
    final Scanner scanner = new Scanner(System.in);

    final int n = scanner.nextInt();
    final int l = scanner.nextInt();
    scanner.nextLine();
    String[] s = new String[n];
    for (int i = 0; i < n; i++) {
      s[i] = scanner.nextLine();
    }

    System.out.println(Arrays.stream(s).sorted().collect(Collectors.joining()));
  }
}
/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