mongodb实现对对某列求和SUM

mongodb数据如下：

db.cpc_0103.find()

{ "a" : 8, "b" : "108177_200701010000", "c" : 108177, "d" : 180, "e" : 1404917, "f" : "U盘", "g" : 184543, "h" : "U盘网上批发市场", "i" : 90, "j" : 3, "k" : 3, "l" : "", "m" : "2007-01-01 00:00:00", "n" : "", "o" : "", "p" : -1, "q" : "--", "r" : 0, "s" : "" }

......

......

如何对其中的一列数据SUM，

xreport-cpc:PRIMARY> var reduce = function(key, values){

... ...     var count=0;

... ...     values.forEach(function(v) {

... ...        count+=v.p;

... ...     });

... ...     return {count: count};

... ... };

xreport-cpc:PRIMARY> var reduce = function(key, values){

... var count=0;

... values.forEach(function(v) {

... count+=v.cm;

... });

... return {count: count};

... };

xreport-cpc:PRIMARY> var s = db.c.find();

xreport-cpc:PRIMARY> var s = db.test.find();

xreport-cpc:PRIMARY> reduce("total_cm", s);

执行结果如下：

{ "count" : 20328860 }