code: def G_calc(item1, item2):
      """
      Calculate G-measure & G-mean.
  
      :param item1: True positive rate (TPR) or True negative rate (TNR) or Positive predictive value (PPV)
      :type item1: float
      :param item2: True positive rate (TPR) or True negative rate (TNR) or Positive predictive value (PPV)
      :type item2: float
      :return: G-measure or G-mean as float
      """
      try:
          result = math.sqrt(item1 * item2)
          return result
      except (TypeError, ValueError):
          return "None"
  
imports: from __future__ import division
  import math
  from .pycm_util import normal_quantile
  from .pycm_interpret import *
  from .pycm_param import CLASS_PARAMS
label_desc: Use image node_id 18 for calling a global function with example usage: G_calc(item1, item2) and returns: result, str