Как сделать выбор в kivy?Чтобы при нажатии на кнопку добавлялись баллы

from kivymd.uix.label import MDLabel, MDIcon
from kivymd.app import MDApp
from kivymd.uix.button import MDFloatingActionButton, MDFlatButton , MDIconButton , MDRectangleFlatButton
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.lang.builder import Builder


screen_helper = """
ScreenManager:
    MenuScreen:
    FirstPage:
<MenuScreen>:
    name: 'menu'
    MDRectangleFlatButton:
        text: 'Start'
        pos_hint: {'center_x':0.5,'center_y':0.6}
        on_press: root.manager.current = 'page'
<FirstPage>
    name:'page'
       MDRectangleFlatButton:
        text: 'Back'
        pos_hint: {'center_x':0.9,'center_y':0.9}
        on_press: root.manager.current = 'menu'
"""
class FirstPage(Screen):
    def build(self):
        label1 = MDLabel(text="Самая большая территория в мире? \n1 Россия \n2 Америка \n3 Япония \n4 Китай",
                         pos_hint={'center_x': 0.65, 'center_y': 0.7}, theme_text_color="Error",
                        font_style="Subtitle2")
        self.test_result = MDLabel(text='Баллы:0', #вывод баллов
                              pos_hint = {'center_x' : 0.9,'center_y': 0.9 }, theme_text_color="Primary" ,
                        font_style ="Subtitle2")
        self.theme_cls.primary_palette = "DeepOrange"
        self.theme_cls.primary_hue = "A700"
        self.theme_cls.theme_style = "Dark"
        screen = Screen()
        self.btn_flat = MDRectangleFlatButton(text='1',
                                              pos_hint={'center_x': 0.2, 'center_y': 0.35})
        self.btn_flat1 = MDRectangleFlatButton(text='2',
                                               pos_hint={'center_x': 0.2, 'center_y': 0.25})
        self.btn_flat2 = MDRectangleFlatButton(text='3',
                                               pos_hint={'center_x': 0.8, 'center_y': 0.35})
        self.btn_flat3 = MDRectangleFlatButton(text='4',
                                               pos_hint={'center_x': 0.8, 'center_y': 0.25}) #кнопки выбора

class MenuScreen(Screen):
    pass

class ProfileScreen(Screen):
    pass

class UploadScreen(Screen):
    pass

class FirstScreen(Screen):
    pass

class DemoApp(MDApp):       #постройка всего кода
    def build(self):

        screen.add_widget(label1)
        screen.add_widget(self.btn_flat)
        screen.add_widget(self.btn_flat1)
        screen.add_widget(self.btn_flat2)
        screen.add_widget(self.btn_flat3)
        screen.add_widget(self.test_result)
        screen = Builder.load_string(screen_helper)
        return screen


sm = ScreenManager()                  #скрины добавление
sm.add_widget(MenuScreen(name='menu')) 
sm.add_widget(ProfileScreen(name='profile'))
sm.add.widget(FirstPage(name='page'))

DemoApp().run()