Add internal table lines

Hi,

I have this situation:

itab1: bukrs hkont kosar gjahr monat actual plan

........ bukrs hkont kosar gjahr 01 actual plan

........ bukrs hkont kosar gjahr 02 actual plan

........ bukrs hkont kosar gjahr 03 actual plan

itab2: bukrs kosar mactual mplan ytdactual ytdplan

Say monat = 3, I need to get mactual, mplan, ytdactual, ytdplan from itab1.

mactual, mplan would be the record where monat = 03.

ytdactual, ytdplan would be the sum of mactual, mplan for monat 01 to 03.

How do i do this? Sample code will be really helpful.

Thanks.