input: Write a python function to calculate the distance between two points.
  ```python
  class Point:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
  def distance(p1, p2):
      import math
      return math.sqrt((p1.x - p2.x)**2 + (p1.y - p2.y)**2)
  
  assert distance(Point(0, 0), Point(3, 4)) == 5
  assert distance(Point(1, 1), Point(4, 5)) == 5
  assert distance(Point(-1, -1), Point(-4, -5)) == 5
output: class Point:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
  def distance(p1, p2):
      import math
      return math.sqrt((p1.x - p2.x)**2 + (p1.y - p2.y)**2)