public static void Main(string[] args)
{
Person person = new Person();
person.SayHello();
Person student = new Student();
student.SetAge(_studentAge);
student.SayHello();
((Student)student).ShowAge();
Person teacher = new Teacher();
teacher.SetAge(_teacherAge);
((Teacher)teacher).Subject = _teacherSubject;
teacher.SayHello();
((Teacher)teacher).Explain();
Console.ReadKey();
}