input: Write a function to print the absolute difference between two given numbers. If any of the numbers are negative, the function should still return the absolute difference.
  assert absolute_difference(9, 5)==4
  assert absolute_difference(-9, 5)==14
  assert absolute_difference(9, -5)==14
  assert absolute_difference(-9, -5)==4
output: def absolute_difference(a, b):
      return abs(a - b)