Quadratic equation program in C# : Could you tell me what's wrong with my code for calculating quadratic equation?
Don't be cringed out on the way of my work, because i'm in still very new to the program. 
    
    class Program
    {
        static void Main(string[] args)
        {
            double a, b, c, x1, x2, x, D;
            String A;
            String B;
            String C;
            Console.Write("a=");
            A = Console.ReadLine(); 
            Console.Write("b=");
            B = Console.ReadLine();
            Console.Write("c=");
            C = Console.ReadLine();
            a = Convert.ToDouble(A);
            b = Convert.ToDouble(B);
            c = Convert.ToDouble(C);
            D =(b * b - 4 * a * c);
            if (D > 0)
            {
                x1 = (-b + Math.Sqrt(D)) / (2 * a);
                x2 = (-b - Math.Sqrt(D)) / (2 * a);
                Console.WriteLine("x1=" + x1);
                Console.WriteLine("x2=" + x2);
            }
            else
                if (D < 0)
                {
                    D = -D;
                    x1 = (-b + Math.Sqrt(D)) / (2 * a);
                    x2 = (-b - Math.Sqrt(D)) / (2 * a);
                }
                else
                {
                    x = (-b / (2 * a));
                    Console.WriteLine("x=" + x);
                }
            Console.ReadKey();

        }
    }