Update user balance by taking the sum between value and input value

I have a model, that extends User. In that model I have an integer field, called user_balance. When I put the form with it to a template, I can update balance with user_profile.user_balance field. But I don't want to just update balance. I need the sum between value, that already exist in balance, and value, that user can input... How can I do that? Am I need an additional fields in models? Or I just need something in my view?

My userprofile model:

class UserProfile(models.Model):  
    user = models.OneToOneField(User)
    user_picture = models.ImageField(upload_to='users', blank=False, null=False, default='users/big-avatar.jpg')
    user_balance = models.IntegerField(default=0)

User.profile = property(lambda u: UserProfile.objects.get_or_create(user=u) [0])

My forms.py

class UserForm(forms.ModelForm):
    class Meta:
        model = User
        first_name = forms.CharField(label='')
        fields = ('first_name', 'last_name', 'email', )
        widgets = {
            'first_name': forms.TextInput(attrs={'placeholder': 'First Name', 'required': True}),
            'last_name': forms.TextInput(attrs={'placeholder': 'Last Name', 'required': True}),
            'email': forms.TextInput(attrs={'placeholder': 'E-Mail', 'required': True}),
        }

class UserProfileForm(forms.ModelForm):
    class Meta:
        model = UserProfile
        fields = ('user_picture', 'user_balance')
        widgets = {
            'user_picture': forms.FileInput(attrs={'class': 'upload_img'}),
        }

And my views.py

def userprofile(request, username):
    u = User.objects.get(username=username)
    if request.POST:
        user_form = UserForm(request.POST, instance=request.user)
        user_profile = UserProfileForm(request.POST, request.FILES, instance=request.user.profile)
        if user_form.is_valid():
            user_form.save()
        if user_profile.is_valid():
            user_profile.save()
    else:
        user_form = UserForm(instance=request.user,
            initial={
                'first_name': request.user.first_name,
                'last_name': request.user.last_name,
                'email': request.user.email,
            })
        user = request.user
        profile = user.profile
        user_profile = UserProfileForm(instance=profile)

    return render_to_response('profile.html', {'user_form': user_form, 'user_profile': user_profile}, context_instance=RequestContext(request))