input: Write a function to complete the Python class so that it can calculate the sum of two integers. The class should initialize with two integers and have a method to return their sum. 
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  assert Sum(3, 4).add()==7
  assert Sum(-1, 5).add()==4
  assert Sum(0, 0).add()==0
  assert Sum(10, -3).add()==7
  assert Sum(7, -7).add()==0
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y