input: Write a recursive Python function to generate Fibonacci numbers. The function should return the n-th Fibonacci number. If n is 1 or 2, return 1. Otherwise, return the sum of the (n-1)th and (n-2)th Fibonacci numbers.
  assert fibonacci(10)==55
  assert fibonacci(1)==1
  assert fibonacci(2)==1
  assert fibonacci(5)==5
  assert fibonacci(0)==None
output: def fibonacci(n):
      if n == 1 or n == 2:
          return 1
      elif n <= 0:
          return None
      else:
          return fibonacci(n-1) + fibonacci(n-2)