func_code: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      elif n > 1:
          x = 0
          y = 1
          z = 2
          for n in range(3, n):
              x = y + z
              y = z
              z = x
          return x
      else:
          retrun - 1