add values of a column for the start of each month

[#1]

Hi, how should the script be like if I want to add up the balance(refer below) for 1 Jan 14 and 1 Feb 14, 1 Feb 14 and 1 Mar 14, etc,...?

itemnum	balance	month
2	3	1 Jan-14
2	10	1 Feb-14
2	6	1 Mar-14
2	4	1 Apr-14
2	8	1 May-14

Thanks.

[#2]

sorry i can't open the file. I'm using personal edition. how about screenshot? thanks.

[tresesco]

Expression you got, then be careful of the sort order:

Update: First expression =balance

               Sec expr= as above