title: Efficient calculation of Fibonacci series
body: <p>I'm working on a <a href="https://projecteuler.net" rel="noreferrer">Project Euler</a> problem: the one about the sum of the even Fibonacci numbers. </p>
  
  <p>My code:</p>
  
  <pre><code>def Fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return Fibonacci(n-1) + Fibonacci(n-2)
  
  list1 = [x for x in range(39)]
  list2 = [i for i in list1 if Fibonacci(i) % 2 == 0]
  </code></pre>
  
  <p>The problem's solution can be easily found by printing sum(list2). However, it is taking a lot of time to come up with the list2 I'm guessing. Is there any way to make this faster? Or is it okay even this way... </p>
  
  <p>(the problem: By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.) </p>