Small and simple code required

Hi

I have following requirement.

Internal table ITAB1 contains following values(ITAB1 is Sorted on F1,F2,F3)

F1 F2 F3 F4

-

---

1 2 2 10

1 2 2 20

1 2 2 15

1 3 4 18

1 3 4 2

2 3 5 10

And Resultant ITAB2 must contain below values

F1 F2 F3 F4

-

---

1 2 2 45

1 3 4 20

2 3 5 10

(i.e The column F4 contains SUM of records having similar F1 F2 and F3)

Can any body provide me with the code?

Reward points will be surely given..

Thanks