input: Write a function to calculate the nth Fibonacci number. If n is 0, return 0, and if n is 1, return 1. For any other n, return the sum of the Fibonacci numbers of n-1 and n-2. If the input is invalid, return None.
  def fibonacci(n):
   if n == 0:
   return 0
   elif n == 1:
   return 1
   else:
   return fibonacci(n - 1) + fibonacci(n - 2)
   
  assert fibonacci(6) == 8
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  assert fibonacci(-1) == None
output: def fibonacci(n):
      if n < 0:
          return None
      elif n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)