Unable to find aggregation builder need help

use sensor
db.data.aggregate([
{
“$unwind”: “$turnstile”
},
{
“$group”: {
“_id”: 0,
“sum”: {
“$sum”: {
$convert:{
input: {$trim: {input: “$count”, chars: “XFER”}},
to: “int”
},
“$count”
},
}
}
}
])

still got this
_id: 0
sum: 0
and it is not the correct answer please someone can analyse my query and tell me what i’m doing wrong?