input: def fibonacci(n):
      """ Calculate the Fibonacci number at the given position n.
      >>> fibonacci(10)
      55
      >>> fibonacci(5)
      5
      """
      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