using System;

public class Program

{

    public static void Main()

    {

        var res = ClosestFib(15);

        Console.WriteLine(res);

    }

    public static int ClosestFib(int num)

    {

        if (num == 0) return 0;

        if (num == 1) return 0;

        int x = 0; int y = 0;

        int i = 0;

        while(true)

        {

            i++;

            var fib = Fib(i);

            Console.WriteLine($"Fib{i} = {fib}");

            if (fib < num)

                x = fib;

            else

            {

                y = fib; 

                break;

            }

        }

        Console.WriteLine($"x = {x}, y= {y}");

        return Math.Min(x, y);

    }

    public static int Fib(int n) {

        if (n == 1)

            return 1;

        if (n==0)

            return 0;

        return Fib(n-1) + Fib(n-2);

    }

}

​