input: 
  from typing import Union
  
  def Fibonacci(n: int) -> Union[int, str]:
      """
      Compute the nth Fibonacci number. If n is less than 0, return an error message.
      >>> Fibonacci(5)
      5
      >>> Fibonacci(10)
      55
      >>> Fibonacci(-1)
      "Incorrect input"
      """
      if n < 0:
          return "Incorrect input"
      elif n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return Fibonacci(n - 1) + Fibonacci(n - 2)
  
output:     if n < 0:
          return "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