using System;

public class Program

{

    public static int Fib(int position)

    {

        var state = new int[]{0,0};

        for(var i = 0; i < position; i++)

        {

            var sum = state[0] + state[1];

            state[0] = state[1];

            if(sum == 0)

            {

                sum = 1;

            }

            state[1] = sum;

        }

        return state[1];

    }

​

​

    public static void Main(string[] args)

    {

        void TestFib(int position)

        {

            Console.WriteLine($"Fib({position}) is {Fib(position)}");

        }

​

        TestFib(0); // should return 0

        TestFib(1); // should return 1

        TestFib(2); // should return 1

        TestFib(3); // should return 2

        TestFib(4); // should return 3

        TestFib(5); // should return 5

​

    }

}