Loops - Fibonacci elements sum

using System;
using System.Numerics;

class Fibonacci
{
    static void Main(string[] args)
    {

        int lastMember = 100;
        bool successParse;
        BigInteger firstNumber = 0;
        BigInteger secondNumber = 1;
        BigInteger sum = 0;
        BigInteger sumFibonacciElements=firstNumber+secondNumber;

        do
        {
            Console.Write("Enter the last element: ");
            successParse = int.TryParse(Console.ReadLine(), out lastMember);
        } while (!successParse);


        for (int i = 2; i < lastMember; i++)
        {
            sum = firstNumber + secondNumber;
            firstNumber = secondNumber;
            secondNumber = sum;
            //Console.Write("{0}, ", sum);
            sumFibonacciElements = sumFibonacciElements + sum;
        }
        Console.WriteLine();
        Console.WriteLine("The sum is: {0}", sumFibonacciElements);

    }
}