input: Write a function to calculate the distance between two points. The function should take four parameters: x1, y1 for the first point and x2, y2 for the second point, and return the Euclidean distance between them.
  
  import math
  
  def distance_between_points(x1, y1, x2, y2):
      return math.sqrt((x2 - x1)**2 + (y2 - y1)**2)
  
  assert distance_between_points(2, 7, 4, 9) == math.sqrt(8)
  assert distance_between_points(0, 0, 3, 4) == 5.0
  assert distance_between_points(-1, -1, 2, 3) == math.sqrt(25)
  assert distance_between_points(1, 2, 1, 2) == 0.0
output: import math
  
  def distance_between_points(x1, y1, x2, y2):
      return math.sqrt((x2 - x1)**2 + (y2 - y1)**2)