import math


def G_calc(item1, item2):
    """
    Calculate G-measure & G-mean.

    :param item1: PPV or TPR or TNR
    :type item1 : float
    :param item2: PPV or TPR or TNR
    :type item2 : float
    :return: G-measure or G-mean as float
    """
    try:
        result = math.sqrt(item1 * item2)
        return result
    except Exception:
        return "None"