input: Write a python function to add two numbers using a class and return the result.
  assert adder(1, 2).__add__() == 3
  assert adder(10, 20).__add__() == 30
  assert adder(-5, 5).__add__() == 0
output: class adder:
      def __init__(self, a, b):
          self.a = a
          self.b = b
  
      def __add__(self):
          return self.a + self.b