using System;

public class Program

{

    public static void Main()

    {

        INumber zero = Number.Zero;

        INumber two = Number.One.Increment();

​

        Console.WriteLine(zero.Increment());

        Console.WriteLine(two.Decrement());

    }

}

​

public interface INumber

{

    int Value { get; }

​

    INumber Negate();

​

    INumber Increment();

​

    INumber Decrement();

​

    INumber Add(INumber number);

​

    INumber Subtract(INumber number);

}

​

public abstract class Number : INumber

{

    public static INumber Zero { get; } = new ZeroNumber();

​

    public static INumber One { get; } = new OneNumber();

​

    public int Value { get; }

​

    protected Number(int value) => this.Value = value;

​

    protected Number(INumber number)

        : this(number?.Value ?? throw new ArgumentNullException(nameof(number)))

    {

        // Intentionally empty.

    }

​

    public override string ToString() => this.Value.ToString();

​

    public INumber Negate() => new OtherNumber(-this.Value);

​

    public INumber Increment()

    {

        int value = this.Value;

​

        return new OtherNumber(++value);

    }

​

    public INumber Decrement()

    {

        int value = this.Value;

​

        return new OtherNumber(--value);

    }

​

    public INumber Add(INumber number) => new OtherNumber(this.Value + number.Value);

​

    public INumber Subtract(INumber number) => this.Add(number.Negate());

​

    private sealed class ZeroNumber : Number

    {

        public ZeroNumber(): base(0) { }

    }

​

    private sealed class OneNumber : Number

    {

        public OneNumber(): base(Zero.Increment()) { }

    }

​

    private sealed class OtherNumber : Number

    {

        public OtherNumber(int value): base(value) { }

    }

}