how to save in inline form total sum

Hi, i need sum inlines field but i no have idea

example for more clean, i have 2 models

in models.py i have:

class ModelA(models.Model):

    field1 = charfield(...)

    field2 = integeterfield()

    .....

CHOICE_TYPE = ((1,"option1"),(2,"option2"),(3,"option3"))

class ModelB(models.Model):

    choice = models.Integerfield(choices='CHOICE_TYPE')

    men = models.IntegerField()

    women = models.IntegerField()

    total_men = integerField(editable=False)

    total_women = integerField(editable=False)

    fk_model_a = fk(ModelA)

    def save(self, *args, **kwargs): #this is for sum all 3 data put in admin inlines

        self.total_men += self.men

        self.total_women += self.women

        super(ModelB, self).save(*args, **kwargs)

in admin.py :

class ModelBInline(admin.TabularInline):

    model = ModelB

    extra = 3

    max_num = 3

class ModelAAdmin(admin.ModelAdmin)

    inlines = [ModelBInline]

admin.site.register(ModelA, ModelAAdmin)

but this override save not working, this error when save datas

unsupported operand type(s) for +=: 'NoneType' and 'int'

and the line error is this:   self.total_men += self.men

any idea for this working.

Cheers

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