import math


def dInd_calc(TNR, TPR):
    """
    Calculate dInd (Distance index).

    :param TNR: specificity or true negative rate
    :type TNR : float
    :param TPR: sensitivity, recall, hit rate, or true positive rate
    :type TPR : float
    :return: dInd as float
    """
    try:
        result = math.sqrt(((1 - TNR)**2) + ((1 - TPR)**2))
        return result
    except Exception:
        return "None"