input: Write a function to calculate the absolute difference between two given numbers.
  assert absolute_difference(9, 5)==4
  assert absolute_difference(10, 20)==10
  assert absolute_difference(-5, 5)==10
  assert absolute_difference(-10, -20)==10
  assert absolute_difference(0, 0)==0
output: def absolute_difference(a, b):
      return abs(a - b)