Mongo “top keyword movers” and absolute sum of subtractions

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MongoDB

I have been trying to calculate the "top keyword movers". For example, the following documents in a date range from 2013-12-19 to 2013-12-23:

{
    "domain" : "www.mytestingdomain.com",
    "keyword" : "elegant",
    "google_rank" : 10,
    "date" : ISODate("2013-12-19T00:00:00Z")
}
{
    "domain" : "www.mytestingdomain.com",
    "keyword" : "elegant",
    "google_rank" : 5,
    "date" : ISODate("2013-12-20T00:00:00Z")
}
{
    "domain" : "www.mytestingdomain.com",
    "keyword" : "elegant",
    "google_rank" : 1,
    "date" : ISODate("2013-12-21T00:00:00Z")
}
{
    "domain" : "www.mytestingdomain.com",
    "keyword" : "elegant",
    "google_rank" : 5,
    "date" : ISODate("2013-12-22T00:00:00Z")
}
{
    "domain" : "www.mytestingdomain.com",
    "keyword" : "elegant",
    "google_rank" : 1,
    "date" : ISODate("2013-12-23T00:00:00Z")
}

Displays how a particular keyword "elegant" has moved in this specific date range.

My question (Using Mongo Aggregate framework):

There is a way to calculate this movements? For example:

10 - 5 = 5
5 - 1 = 4
1 - 5 = -4 --> abs --> 4
5 - 1 = 4
---------------------------

absolute total: 5 + 4 + 4 + 4 = 17 --> in this case "seventeen" is the absolute sum of moves that I am looking for.