Django form that varies according to another form

Question

How do I create a form in Jango of an object and that the price of the object varies according to the size of the object without having to update the page.

#py.models

class Pizza(models.Model):
    SIZE_OPTIONS = (
        ('30', 'Pequena'),
        ('35', 'Média'),
        ('40', 'Grande'),

    id = models.AutoField(primary_key=True)
    flavor = models.ForeignKey(Flavor, default=None, on_delete=models.CASCADE)
    size = models.CharField(max_length=2, choices=SIZE_OPTIONS)

#form py.

class OrderForm(forms.ModelForm):
class Meta:
    model = Pizza
    fields = ['flavor', 'size']

#html template.

{% extends 'base.html' %}

{% block content %}
    {{ form.as_ul }}
    {{ price }}
{% endblock %}

I want the person to select the size and price upgrade alone.