Ошибка в программе в python

import sys
from PyQt5 import QtCore, QtGui, QtWidgets
from gla1 import Ui_Dialog



app = QtWidgets.QApplication(sys.argv)
Dialog = QtWidgets.QDialog()
ui = Ui_Dialog()
ui.setupUi(Dialog)
s = 5
Dialog.show()


def plu():
    s = s + 1
    ui.label.setText(f'Сахар +')
 def mun():
    s = s - 1
ui.label.setText('Сахар ' + str(s))




ui.pushButton.clicked.connect(mun)
ui.pushButton_9.clicked.connect(plu)


sys.exit(app.exec_())

Выдает ошибка:

Traceback (most recent call last):
File "main.py", line 16, in plu
   s = s + 1
UnboundLocalError: local variable 's' referenced before assignmen