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