N-th member of a number

using System;

namespace N_th_member_of_a_number
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Please enter the number: ");
            int a = int.Parse(Console.ReadLine());

            int iterativeProduct = iterativeVersion(a);

            static int iterativeVersion(int a)
            {

                int a1 = 2;
                int a2 = 4;
                int a3 = 6;
                int result = 0;

                if (a == 1)
                    return 2;
                if (a == 2)
                    return 4;
                if (a == 3)
                    return 6;

                for (int i = 4; i <= a; i++)
                {
                    result = 3 * a1 + 4 * a2 - 7 * a3;
                    a1 = a2;
                    a2 = a3;
                    a3 = result;
                }
                return result;
            }
            Console.WriteLine($"Iterative result is: {iterativeProduct}");
            Console.WriteLine();

            int recursiveProduct = recursiveVersion(a);

            static int recursiveVersion(int a)
            {
                if (a == 1)
                    return 2;
                if (a == 2)
                    return 4;
                if (a == 3)
                    return 6;

                return 3 * recursiveVersion(a - 3) + 4 * recursiveVersion(a - 2) - 7 * recursiveVersion(a - 1);
            }
            Console.WriteLine($"Recursive result is: {recursiveProduct}");
        }
    }
}