mongodb中$sum:1 后面的1是什么意思

{

    "_id" : "GuqXmAkkARqhBDqhy",

    "beatmapset_id" : "342537",

    "version" : "MX",

    "diff_approach" : "5",

    "artist" : "Yousei Teikoku",

    "title" : "Kokou no Sousei",

    "difficultyrating" : "3.5552737712860107"

}

{

    "_id" : "oHLT7KqsB7bztBGvu",

    "beatmapset_id" : "342537",

    "version" : "HD",

    "diff_approach" : "5",

    "artist" : "Yousei Teikoku",

    "title" : "Kokou no Sousei",

    "difficultyrating" : "2.7515676021575928"

}

{

    "_id" : "GbotZfrPEwW69FkGD",

    "beatmapset_id" : "342537",

    "version" : "NM",

    "diff_approach" : "5",

    "artist" : "Yousei Teikoku",

    "title" : "Kokou no Sousei",

    "difficultyrating" : "0"

}

db.getCollection('dup_case').aggregate(

[

{$group:{

    _id:{diff_approach:'$diff_approach'},

    count:{$sum:2}

    }},

{$match:{count:{$gt:1}}}

]

)