mongodb - how to return 0 for count of match that doesn't exist -

so i'm using aggregation match if value in array group values:

db.person.aggregate([     {        $match: {"city":{ $in: ["new york", "denver"]}}     },{        $group: {_id: "$city", total : {$sum:1}}     } ])  {_id:"new york", total:5} {_id:"denver", total:2} 

this works in case city in values return total:0 in case city isn't in set. ie:

["new york", "denver", "orlando"]   {_id:"new york", total:5} {_id:"denver", total:2} {_id:"orlando", total:0} 

is possible or should check results in code , append 0 value results results?