Error: "an object reference is required for the non-static field, method or property..."
<p>Im creating an application on C#. Its function is to evualuate if a given  is prime and if the same swapped number is prime as well. When I build my solution on Visual Studio, it says that "an object reference is required for the non-static field, method or property...". Im having this problem with the "volteado" and "siprimo" methods.
Can you tell me where is the problem and how i can fix it? thank you! </p>

<pre><code>namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Write a number: ");
            long a= Convert.ToInt64(Console.ReadLine()); // a is the number given by the user


            long av = volteado(a); // av is "a" but swapped

            if (siprimo(a) == false &amp;&amp; siprimo(av) == false) 
                Console.WriteLine("Both original and swapped numbers are prime.");
            else
                Console.WriteLine("One of the numbers isnt prime.");
            Console.ReadLine();


        }
        private bool siprimo(long a)
        {// evaluate if the received number is prime
            bool sp = true;
            for (long k = 2; k &lt;= a / 2; k++)
                if (a % k == 0) sp = false;
            return sp;
        }
        private long volteado(long a)
        {// swap the received number
            long v = 0;
            while (a &gt; 0)
            {
                v = 10 * v + a % 10;
                a /= 10;
            }
            return v;
        }
    }
}
</code></pre>
