input: def fibonacci(n: int) -> int:
      """Calculate the nth number in the Fibonacci sequence.
      The Fibonacci sequence is a series of numbers where a number is 
      the sum of the two preceding ones, starting from 0 and 1.
      
      >>> fibonacci(0)
      0
      >>> fibonacci(1)
      1
      >>> fibonacci(9)
      34
      """
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