using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        int []list_Number = {1,4,10,11,7,20,21,13};

        LinkedList<int> linkedList_Number = new LinkedList<int>(list_Number);

        LinkedList<int> prime_Number_LinkedList = new LinkedList<int>();

        foreach (int is_primeNumber in linkedList_Number)

        {

            if (is_primeNumber > 2){    

                int flag = 0;

                for (int i = 2; i <= Math.Sqrt(is_primeNumber); i++){

                    if (is_primeNumber % i == 0){

                        flag++;

                    }

                }

                if (flag == 0){

                    prime_Number_LinkedList.AddLast(is_primeNumber);

                }

            }

        }

        String prime_Number = String.Join(", ", prime_Number_LinkedList);

        Console.WriteLine(prime_Number);

    }

}