Only certain spheres., C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        static double a = 0.0, b = 0.0;
        static void input ()
        {
            Console.WriteLine("Escriba su nùmero: ");
            a=double.Parse(Console.ReadLine());
        }
        static void cal ()
        {
            b=(4*3.141*(a*a*a))/3;
        }
        static void val ()
        {
            if (a%5==0)
            {
                Console.WriteLine("Tu resultado es: "+b.ToString());
            }
            else
            {
               Console.WriteLine("FATAL ERROR! PLEASE RESTART COMPUTER ");
            }
        }
        public static void Main(string[] args)
        {
            input ();
            cal ();
            val ();
        }
    }
}

Show compiler warnings [ - ] Show input
Compilation time: 0,12 sec, absolute running time: 0,11 sec, cpu time: 0,11 sec, average memory usage: 12 Mb, average nr of threads: 3