using System;

using System.Diagnostics;

public class Program

{

    public static void Main()

    {

        var watch = new Stopwatch();

        watch.Start();

        Console.WriteLine(default(Fib<Zero>));

        Console.WriteLine(default(Fib<Succ<Zero>>));

        Console.WriteLine(default(Fib<Succ<Succ<Zero>>>));

        Console.WriteLine(default(Fib<Succ<Succ<Succ<Zero>>>>));

        Console.WriteLine(default(Fib<Succ<Succ<Succ<Succ<Succ<Zero>>>>>>));

        Console.WriteLine(default(Fib<Succ<Succ<Succ<Succ<Succ<Succ<Zero>>>>>>>));

        Console.WriteLine(watch.Elapsed);

        watch.Stop();

    }

}

​

interface INumber

{

    int Value { get; }

}

​

struct Zero : INumber

{

    public int Value => 0;

}

​

struct Succ<TPrev> : INumber where TPrev : INumber

{

    public int Value => default(TPrev).Value + 1;

}

​

struct Prev<TSucc> : INumber where TSucc : INumber

{

    public int Value => default(TSucc).Value -1;

}

​

struct Minus<TLeft, TRight> : INumber where TLeft : INumber where TRight : INumber

{

    public int Value

    {

        get

        {

            if (default(TRight).Value == 0)

                return default(TLeft).Value;

            return default(Minus<Prev<TLeft>, Prev<TRight>>).Value;     

        }

​

    }

}

​

struct Fib<T> : INumber where T : INumber

{

    public int Value

    {

        get

        {

            if (default(T).Value == 0)

                return 0;

            else if (default(T).Value == 1)

                return 1;

            else 

                return default(Fib<Minus<T, Succ<Zero>>>).Value + default(Fib<Minus<T, Succ<Succ<Zero>>>>).Value;

        }

    }

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

}