fibonacci-2013-05-31

using System;
using System.Numerics;

class SumNFibonacciSequence
{
    static void Main()
    {
        Console.Write("Please enter how many N members you want to calculate: ");
        int n = int.Parse(Console.ReadLine());

        if(n < 1)
        {
            Console.WriteLine("Invalid input!");
            return;
        }

        BigInteger sum;

        if (n > 2)
        {
            BigInteger number1 = 0;
            BigInteger number2 = 1;
            sum = 1;

            for (int i = 3; i <= n; i++)
            {
                number2 = number1 + number2;
                number1 = number2 - number1;
                sum += number2;
            }
        }
        else if (n == 2)
        {
            sum = 1;
        }
        else if (n == 1)
        {
            sum = 0;
        }

        Console.WriteLine("Sum = {0}", sum);
    }
}