input: def fibonacci(n):
      """
      Generate a Fibonacci number recursively by given n.
      
      >>> fibonacci(1)
      1
      >>> fibonacci(2)
      1
      >>> fibonacci(10)
      55
      >>> fibonacci(6)
      8
      """
output:     if n <= 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)