How to use the flasgger.fields.Int function in flasgger

spec = APISpec(
    title='Flasger Petstore',
    version='1.0.10',
    openapi_version='2.0',
    plugins=[
        FlaskPlugin(),
        MarshmallowPlugin(),
    ],
)

app = Flask(__name__)


# Optional marshmallow support
class CategorySchema(Schema):
    id = fields.Int()
    name = fields.Str(required=True)


class PetSchema(Schema):
    category = fields.Nested(CategorySchema, many=True)
    name = fields.Str()


@app.route('/random')
def random_pet():
    """
    A cute furry animal endpoint.
    Get a random pet
    ---
    description: Get a random pet
    responses:

or you can specify:
    @swag_from('file.yml',
               validation=True,
               definition='User',
               data=lambda: request.json,  # any callable
               )
    """
    data = request.json
    return jsonify(data)


class User(Schema):
    username = fields.Str(required=True, default="Sirius Black")
    # wrong default "180" to force validation error
    age = fields.Int(required=True, min=18, default="180")
    tags = fields.List(fields.Str(), default=["wizard", "hogwarts", "dead"])


class UserPostView(SwaggerView):
    tags = ['users']
    parameters = User
    responses = {
        200: {
            'description': 'A single user item',
            'schema': User
        }
    }
    validation = True

    def post(self):
        """

spec = APISpec(
    title='Flasger Petstore',
    version='1.0.10',
    openapi_version='2.0',
    plugins=[
        FlaskPlugin(),
        MarshmallowPlugin(),
    ],
)

app = Flask(__name__)


# Optional marshmallow support
class CategorySchema(Schema):
    id = fields.Int()
    name = fields.Str(required=True)


class PetSchema(Schema):
    category = fields.Nested(CategorySchema, many=True)
    name = fields.Str()


@app.route('/random')
def random_pet():
    """
    A cute furry animal endpoint.
    Get a random pet
    ---
    description: Get a random pet
    responses:

from flask import Flask, jsonify, request

from flasgger import Schema, Swagger, SwaggerView, fields

app = Flask(__name__)
app.config['SWAGGER'] = {
    "title": "API using Marshmallow",
    "uiversion": 2
}

swag = Swagger(app)


class User(Schema):
    username = fields.Str(required=True)
    age = fields.Int(required=True, min=18)
    tags = fields.List(fields.Str())


class UserPostView(SwaggerView):

    parameters = User

    # parameters = [
    #     {
    #         'name': 'body',
    #         'in': 'body',
    #         'schema': User
    #     }
    # ]

    responses = {