How To Convert String To Double While Using Aggregation in mongoDB v3.6.5

Question

I want to solve this with Aggregation if in other way possible please do let me know

I Tried This with aggregation :

 db.CLV_MATERIAL_AGING.aggregate([ { $match: {"Material_Number" : 
  "000000000810000010"}}, 
{ 
   $group: {
      _id: 'null', 
      total:{
          $sum:{$convert: {'input': '$MR', 'to': 'double'}}     
     },
      count:{
          $sum:1
     }
 }
}])

Give Me Error :-

  {
    "message" : "Unrecognized expression '$convert'",
     "stack" : "MongoError: Unrecognized expression '$convert'" +
          "at queryCallback 
(/tmp/.mount_nosqlbn5xIBs/app/resources/app.asar/node_modules/mongodb- 
core/lib/cursor.js:247:25)" +
          "at 
/tmp/.mount_nosqlbn5xIBs/app/resources/app.asar/node_modules/mongodb- 
core/lib/connection/pool.js:531:18" +
          "at _combinedTickCallback (internal/process/next_tick.js:131:7)" +
          "at process._tickCallback (internal/process/next_tick.js:180:9)",
 "name" : "MongoError",
 "ok" : 0,
 "errmsg" : "Unrecognized expression '$convert'",
 "code" : 168,
 "codeName" : "InvalidPipelineOperator"
 }

AND $toDouble() Not in mongoDB v3.6.5 i already Tried on mongodb v4 it worked but not in mongodb v3.5.6