input: def Fibonacci(n: int) -> int: 
      """
      Returns the nth Fibonacci number. If the input is less than 0, it prints "Incorrect input" and returns None.
      
      >>> Fibonacci(5)
      5
      >>> Fibonacci(10)
      55
      """
      if n < 0: 
          print("Incorrect input") 
          return None
      elif n == 0: 
          return 0
      elif n == 1: 
          return 1
      else: 
          return Fibonacci(n-1) + Fibonacci(n-2)
output:     a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b