
import java.util.*;

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);
    StringBuilder sb = new StringBuilder();
    for (String s : S) {
      sb.append(s);
    }

    System.out.println(sb.toString());
  }
}
/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