input: 
  def fibonacci(n):
      if n == 1 or n == 2:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)
  
  """
  Generate Fibonacci numbers using a recursive function.
  >>> fibonacci(1)
  1
  >>> fibonacci(2)
  1
  >>> fibonacci(5)
  5
  >>> fibonacci(10)
  55
  """
  
output:     if n > 2:
          a, b = 1, 1
          for _ in range(3, n + 1):
              a, b = b, a + b
          return b