input: 
  def Fibonacci(n):
      """ Calculate the Fibonacci numbers up to a given number n.
      >>> Fibonacci(0)
      0
      >>> Fibonacci(1)
      1
      >>> Fibonacci(5)
      5
      >>> Fibonacci(10)
      55
      """
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return Fibonacci(n-1) + Fibonacci(n-2)
  
output:     a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b