input: Write a function to find the sum of two numbers using a class.
  assert Sum(10, 5).find_sum() == 15
  assert Sum(3, 7).find_sum() == 10
  assert Sum(-1, 1).find_sum() == 0
output: class Sum:
      def __init__(self, a, b):
          self.a = a
          self.b = b
      
      def find_sum(self):
          return self.a + self.b