input: Write a python function to compute the age difference between two persons using a dictionary of ages.
  assert compute_age_difference('John Doe', 'Jane Doe') == 8
  assert compute_age_difference('Jane Doe', 'John Doe') == 8
  assert compute_age_difference('John Doe', 'John Doe') == 0
output: def compute_age_difference(person1, person2):
      ages = {
          'John Doe': 34,
          'Jane Doe': 26,
          'Alice Smith': 30,
          'Bob Johnson': 40
      }
      return abs(ages[person1] - ages[person2])