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 found by 
      adding up the two numbers before it. Starting with 0 and 1, the sequence goes:
      0, 1, 1, 2, 3, 5, 8, 13, 21, and so on.
      >>> fibonacci(9)
      21
      >>> fibonacci(1)
      1
      >>> fibonacci(0)
      0
      >>> fibonacci(5)
      3
      """
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