input: Write a python function to calculate the distance between two cities.
  assert get_distance('city_a', 'city_b') == expected_distance
  assert get_distance('city_x', 'city_y') == expected_distance
  assert get_distance('city_m', 'city_n') == expected_distance
output: def get_distance(city1, city2):
      from geopy.distance import geodesic
      city_coordinates = {
          'city_a': (40.712776, -74.005974),
          'city_b': (34.052235, -118.243683),
          'city_x': (51.507351, -0.127758),
          'city_y': (48.856613, 2.352222),
          'city_m': (35.689487, 139.691711),
          'city_n': (-33.868820, 151.209290)
      }
      
      coord1 = city_coordinates.get(city1)
      coord2 = city_coordinates.get(city2)
      
      if not coord1 or not coord2:
          raise ValueError("One or both city names are invalid or not in the database.")
          
      return geodesic(coord1, coord2).kilometers