input: Write a function to calculate the absolute difference between two given numbers.
  
  assert absolute_difference(9, 5)==4
  assert absolute_difference(3, 8)==5
  assert absolute_difference(-7, 12)==19
  assert absolute_difference(0, 0)==0
  assert absolute_difference(-10, -5)==5
output: def absolute_difference(a, b):
      return abs(a - b)