Ariphmetic Progression

static void Main(string[] args)
        {

            double a, b, c = 0;

            do
            {
                Console.WriteLine("Введите число a");
                a = Convert.ToDouble(Console.ReadLine());
            } while (a <= 0);

            do
            {
                Console.WriteLine("Введите число b");
                b = Convert.ToDouble(Console.ReadLine());
            } while (b <= 0);


            Console.WriteLine("Введите число c");
            double needed = Convert.ToDouble(Console.ReadLine());
            c = needed;
            while (needed != 0) {

                Console.WriteLine("Введите число c");
                needed = Convert.ToDouble(Console.ReadLine());
                if (needed != 0)
                {
                    a = b;
                    b = c;
                    c = needed;
                }
            }
            Console.WriteLine( ariphmeticProgression(a, b, c) );
            Console.ReadKey();
        }

        static bool ariphmeticProgression(double a,double b,double c)
        {
            if( b - a == c - b)
            {
                return true;
            }
            else
            {
                return false;
            }
        }