input: Write a python function to sum two numbers using a class.
  assert Sum(3, 4).add() == 7
  assert Sum(10, 15).add() == 25
  assert Sum(-5, 5).add() == 0
output: class Sum:
      def __init__(self, a, b):
          self.a = a
          self.b = b
  
      def add(self):
          return self.a + self.b