input: Write a function to calculate and return the absolute difference between two given numbers. If one or both of the numbers are negative, the function should still return the absolute difference.
  assert absolute_difference(9, 5)==4
  assert absolute_difference(-3, 7)==10
  assert absolute_difference(0, 0)==0
  assert absolute_difference(10, -10)==20
  assert absolute_difference(-5, -5)==0
output: def absolute_difference(a, b):
      return abs(a - b)