item wise sum

hi experts,

I have an itab where i have

delivno matnr weight

1 A 1000

1 A 500

1 B 2000

i need an output like this

delivno matnr weight

1 A 1500

1 B 2000

sort itab by delivno matnr.

loop at itab.

at new matnr.

sum.

write:/ itab-delivno,itab-matnr,itab-weight.

endat.

endloop.

but it is not working ..what mey be the reason ..

what shall i do to get the output.

regards,

mani