def imageFile_to_number(filename):
    """
    find()方法：查找子字符串，若找到返回从0开始的下标值，若找不到返回-1
    ws0.7_crowding1_n448_Ndisk57.png --> 448
    """
    numberEndIndex = filename.find('_Ndisk')
    filename = filename[18:numberEndIndex]
    return filename