Почему выводит Nan - C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            int a, b, c;
            Console.WriteLine("Enter a,b,c please");
            a = Convert.ToInt32(Console.ReadLine());
            b = Convert.ToInt32(Console.ReadLine());
            c = Convert.ToInt32(Console.ReadLine());
           double d = b * b - 4 * a * c;
            double x1, x2;
            x1 = (-b - Math.Sqrt(d)) / 2 * a;
            x2 = (-b + Math.Sqrt(d)) / 2 * a;
            Console.WriteLine("x1=" + x1 + "x2=" + x2 + "");
        }
    }
}