Database

[{"_id":"series/cogtech/BertonKHS06","type":"Article","title":"Speech Recognition.","pages":{"start":85,"end":107},"year":2006,"booktitle":"SmartKom","url":"db/series/cogtech/54023732.html#BertonKHS06","authors":["Andr? Berton","Alfred Kaltenmeier","Udo Haiber","Olaf Schreiner"]}]

Query

db.collection.aggregate([{$addFields:{average_pages:{$divide:[{$subtract:["$pages.end","$pages.start"]},2]}}}])

Result