input: def fibonacci(n: int) -> int:
      """Calculate the nth Fibonacci number.
      >>> fibonacci(6)
      8
      """
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)
  
  print(fibonacci(6)) // 8
output:     a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b