Error encountered when using lambda to change variable values

I'm trying to calculate the values when one variable has another variable in the formula. I've used the lambda approach as suggested by @Rain366, however, this approach has a limitation and I received this error,

Error CS0200 Property or indexer 'Class3.Storage.b' cannot be assigned to -- it is read only.

I've tried using the get and set approach public static int b { get; set; } => a + 5;, but I believe the method of doing it is wrong. Is there a way to use lambda to write values or possibly merge it with a setter?

What I would like to see is the value of b constantly updating itself (therefore I've used the lambda approach) and the flexibility of editing the value of b by using something like Storage.b++;

Thank you!

Code as shown below.

class Class3
{
public class Storage
    {
        public static int a = 100;
        public static int b => a + 5;
        public static int c;
    }
public static void Main()
    {
        Methods Test = new Methods();
        Console.WriteLine("Original a value: {0}", Storage.a);
        Console.WriteLine("b value: {0}", Storage.b);
        Console.WriteLine("c value: {0}", Storage.c);
        Test.Met1();
        Console.WriteLine("After met1: {0}", Storage.a);
        Console.WriteLine("b value: {0}", Storage.b);
        Console.WriteLine("c value: {0}", Storage.c);
        Test.Met2();
        Console.WriteLine("After met2: {0}", Storage.a);
        Console.WriteLine("b value: {0}", Storage.b);
        Console.WriteLine("c value: {0}", Storage.c);
        Test.Met3();
        Console.WriteLine("After met3: {0}", Storage.a);
        Console.WriteLine("b value: {0}", Storage.b);
        Console.WriteLine("c value: {0}", Storage.c);

        Storage.b += 1;
        Console.WriteLine("b value: {0}", Storage.b);
    }
    public class Methods
    {
        public void Met1()
        {
            Storage.a -= 10;
            Storage.c = Storage.a;
        }
        public void Met2()
        {
            Storage.a -= 10;
            Storage.c = Storage.a;
        }
        public void Met3()
        {
            Console.WriteLine("{0}", Storage.a);
            Storage.c = Storage.a;
            Met1();
            Met2();
            if (Storage.a > 10)
            {
                Met3();
            }
        }
    }
}