07_Fibonacci

using System;
using System.Numerics;

/*
 * 7. Write a program that reads a number N and calculates the sum of the first N members
 * of the sequence of Fibonacci: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, …
 */
class Fibonacci
{
    static void Main()
    {
        int n;
        BigInteger first = 0;
        BigInteger second = 1;
        BigInteger tmp = 0;
        BigInteger sum = 0;

        Console.Write("N = ");
        string strN = Console.ReadLine();

        if (!int.TryParse(strN, out n))
        {
            Console.WriteLine("Invalid number: {0}", strN);
        }
        else
        {
            for (int i = 0; i < n; i++)
            {
                tmp = first;
                first = second;
                second += tmp;

                sum += tmp;
            }

            Console.WriteLine("The sum of the element to N is {0}.", sum);
        }
    }
}