static void Main(string[] args)
{
Person p1 = new Person();
Student s1 = new Student();
Teacher t1 = new Teacher();
p1.Greet();
s1.SetAge(21);
s1.Greet();
s1.ShowAge();
t1.SetAge(30);
t1.Greet();
t1.Explain();
Console.ReadKey();
}