input: def Fibonacci(n: int) -> int:
      """Calculate the nth Fibonacci number.
      If n is less than 0, print an error message and return None.
      If n is 0 or 1, return n.
      For other values of n, recursively calculate the Fibonacci number.
      >>> Fibonacci(0)
      0
      >>> Fibonacci(1)
      1
      >>> Fibonacci(5)
      5
      >>> Fibonacci(10)
      55
      """
output:     if n < 0:
          print("Error: n should be a non-negative integer.")
          return None
      elif n == 0 or n == 1:
          return n
      else:
          return Fibonacci(n - 1) + Fibonacci(n - 2)