def get_npa(fp, tn):
    """
    This functions returns the negative percent agreement.

    :param fp: Number of false positives
    :type fp: int
    :param tn: Number of true negatives
    :type tn: int

    :return: The negative percent agreement, NPA = (tn) / (fp+tn)
    :rtype: float
    """
    if fp + tn == 0:
        return 'NaN'
    else:
        return format(round(100 * (float(tn) / (fp + tn)), 2), '.2f')