06_QuadraticEquation

using System;

class QuadraticEquation
{
    static void Main()
    {
        int a;
        int b;
        int c;
        double x1;
        double x2;

        Console.Write("Please enter the first number: ");
        string firstStr = Console.ReadLine();

        Console.Write("Please enter the second number: ");
        string secondStr = Console.ReadLine();

        Console.Write("Please enter the third number: ");
        string thirdStr = Console.ReadLine();

        if (!int.TryParse(firstStr, out a))
        {
            Console.WriteLine("Invalid number: {0}", firstStr);
        }
        else if (!int.TryParse(secondStr, out b))
        {
            Console.WriteLine("Invalid number: {0}", secondStr);
        }
        else if (!int.TryParse(thirdStr, out c))
        {
            Console.WriteLine("Invalid number: {0}", thirdStr);
        }
        else
        {
            while (a == 0)
            {
                Console.WriteLine("A must be different from 0!");
                string str = Console.ReadLine();

                if (!int.TryParse(str, out a))
                {
                    Console.WriteLine("Invalid number: {0}", str);
                }
            }

            // Calculate the discriminant
            double d = b * b - 4 * a * c;

            if (d < 0)
            {
                Console.WriteLine("The equation doesn't have any real roots.");
            }
            else if (d == 0)
            {
                x1 = (-b) / 2 * a;
                Console.WriteLine("The equation have only one real root: {0}", x1);
            }
            else
            {
                x1 = (-b) + Math.Sqrt(d) / 2 * a;
                x2 = (-b) - Math.Sqrt(d) / 2 * a;
                Console.WriteLine("The real roots are:");
                Console.WriteLine("X1 is {0} and X2 is {1}", x1, x2);
            }
        }
    }
}