Advanced Topics 1. Fibonacci Numbers

// Define a method Fib(n) that calculates the nth Fibonacci number.

namespace _01.FibonacciNumbers
{
    using System;
    using System.Numerics;

    public class FibonacciNumbers
    {
        public static void Main(string[] args)
        {
            checked
            {
                int num;
                do
                {
                    Console.Write("Enter a non-negative Integer number: ");
                }
                while (!int.TryParse(Console.ReadLine(), out num) || num < 0);

                BigInteger n_thFibNum = CalculateN_thFibonacciNumber(num);
                Console.WriteLine("The {0}-th Fibonacci number is: {1} !", num, n_thFibNum);
            }
        }

        private static BigInteger CalculateN_thFibonacciNumber(int num)
        {
            checked
            {
                BigInteger fibNum = 1;
                if (num > 1)
                {
                    BigInteger fibNumBeforePrevious = fibNum;
                    BigInteger fibNumPrevious = fibNum;
                    for (int i = 1; i < num; i++)
                    {
                        fibNum = fibNumBeforePrevious + fibNumPrevious;
                        fibNumBeforePrevious = fibNumPrevious;
                        fibNumPrevious = fibNum;
                    }
                }

                return fibNum;
            }
        }
    }
}