编写一个包含定义、执行的代码块，实现以下功能:根据部门编号变量的值输出本部门所有员工的平均工资。

假设我们有一个简单的员工类（Employee）和一个部门类（Department），每个部门都有一个员工列表（employees）以及一个计算平均工资的方法。以下是Python代码实现：

class Employee:
    def __init__(self, id, name, salary):
        self.id = id
        self.name = name
        self.salary = salary

class Department:
    def __init__(self, department_id):
        self.department_id = department_id
        self.employees = []

    def add_employee(self, employee):
        self.employees.append(employee)

    def average_salary(self):
        if not self.employees:
            return "No employees in this department."
        else:
            total_salary = sum(employee.salary for employee in self.employees)
            return total_salary / len(self.employees)


# 创建示例数据
dept1 = Department(1)
dept1.add_employee(Employee(101, 'Alice', 5000))
dept1.add_employee(Employee(102, 'Bob', 6000))

 dept2 = Department(2)
dept2.add_employee(Employee(201, 'Charlie', 7000))
dept2.add_employee(Employee(202, 'Dave', 8000))

# 根据部门编号获取并计算平均工资
def get_department_average_salary(department_id):
    try:
        target_dept = [dept for dept in departments if dept.department_id == department_id][0]
        return target_dept.average_salary()
    except IndexError:
        return "Department with the specified ID does not exist."

departments = [dept1, dept2]  # 部门集合
department_number = 1  # 示例部门编号
average_salary = get_department_average_salary(department_number)
print(f"The average salary in Department {department_number} is: {average_salary}")