input: 
  def Fibonacci(n: int) -> int: 
      """ Compute the nth Fibonacci number.
      >>> Fibonacci(5)
      5
      >>> Fibonacci(10)
      55
      """
      if n < 0: 
          raise ValueError("Incorrect input") 
      elif n == 0: 
          return 0
      elif n == 1: 
          return 1
      else: 
          return Fibonacci(n-1) + Fibonacci(n-2)
  
output:     if n < 0: 
          raise ValueError("Incorrect input") 
      elif 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