input: Write a python function to create a Student class that inherits from a Person class and has additional method show_rollnum. Use the following test cases to validate the implementation.
  assert s.name == "Jill"
  assert s.rollnum == "15"
  assert isinstance(s, Student)
  assert isinstance(s, Person)
output: class Person:
      def __init__(self, name):
          self.name = name
  
  class Student(Person):
      def __init__(self, name, rollnum):
          super().__init__(name)
          self.rollnum = rollnum
  
      def show_rollnum(self):
          return self.rollnum
  
  # Example usage
  s = Student("Jill", "15")