Only download 1080p when 720p is not available

I currently have my config file setup this way:

tasks:
  tvshows:
    rss: <insertrssfeedhere>
    series:
      720p:
        - modern family
        - brooklyn nine
        - better things
    download: C:\Users\dog2b\flexget\shows

How could I make it so it only downloads 1080p when 720p is not available?

Look into timeframe