input: Write a function to generate Fibonacci numbers using a recursive approach in Python. If the input is 1 or 2, the function should return 1. Otherwise, the function should return the sum of the two preceding Fibonacci numbers. If the input is less than 1, the function should return None.
  assert fibonacci(1)==1
  assert fibonacci(2)==1
  assert fibonacci(10)==55
  assert fibonacci(0)==None
  assert fibonacci(-1)==None
output: def fibonacci(n):
      if n < 1:
          return None
      elif n == 1 or n == 2:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)