Fibonacci Sum

/*
 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, …
Each member of the Fibonacci sequence (except the              first two) is a sum of the previous two members.

 */

using System;
using System.Numerics;

class FibonacciSum
{
    static void Main()
    {
        BigInteger membersOfSequence;
        string invalidInput = "Please enter a valid number greater than 1!" + Environment.NewLine;
        BigInteger currentNumber, nextNumber, previousNumber;

        while (true)
        {
            Console.WriteLine("How many members of sequence of Fibonacci you want to sum? ");
            while (!(BigInteger.TryParse(Console.ReadLine(), out membersOfSequence) && membersOfSequence >= 1))
            {
                Console.WriteLine(invalidInput);
                Console.WriteLine("Enter valid value: ");
            }

            currentNumber = 0;
            nextNumber = 1;
            BigInteger sum = 0;
            for (ulong i = 1; i <= membersOfSequence; i++)
            {
                sum = sum + currentNumber;
                previousNumber = currentNumber;
                currentNumber = nextNumber;
                nextNumber = currentNumber + previousNumber;
            }
            Console.WriteLine();
            Console.WriteLine("The sum of first {0} members of sequence of Fibonacci is: {1}" + Environment.NewLine,membersOfSequence, sum);
        }
    }
}