import java.util.*;
public 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];
    for(int i=0; i<N; i++){
    	S[i] = sc.next();
    }
    Arrays.sort(S);
    StringBuilder string = new StringBuilder();
    for(int i=0; i<N; i++){
    	string.append(S[i]);
    }
    System.out.println(string);
  }
}
/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