class Person:
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name
        self.age = age

    def introduce(self):
        return f"Hi. I'm {self.full_name}. I'm {self.age} years old."

    @property
    def age(self):
        return self.__age

    @age.setter
    def age(self, value):
        if value <= 0:
            raise ValueError("Age is not valid")
        self.__age = value

    @property
    def full_name(self):
        return f"{self.first_name} {self.last_name}"


class Employee(Person):
    def __init__(self, first_name, last_name, age, job_title, salary):
        super().__init__(first_name, last_name, age)
        self.job_title = job_title
        self.salary = salary

    @property
    def job_title(self):
        return self.__job_title

    @job_title.setter
    def job_title(self, value):
        self.__job_title = value

    @property
    def salary(self):
        return self.__salary

    @salary.setter
    def salary(self, value):
        if value < 0:
            raise ValueError("Salary must be greater than zero.")
        self.__salary = value

    def introduce(self):
        introduction = super().introduce()
        introduction += f" I'm a {self.job_title}."
        return introduction


employee = Employee("John", "Doe", 25, "Python Developer", 120000)
print(employee.introduce())