input: def Fibonacci(n: int) -> int:
      """Find the nth Fibonacci number.
      >>> Fibonacci(1)
      1
      >>> Fibonacci(4)
      3
      >>> Fibonacci(8)
      21
      """
      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:
          return Fibonacci(n - 1) + Fibonacci(n - 2)