[rules-users] problem with drules

I have following plan for the free item distribution purchase of 3 items under a same brands will get 1 gift item free purchase of 5 items under a same brands will get 2 gift item free purchase of 10 items under a same brands will get 4 gift item free now if some buyer buys 13 items then he wil get free ites 4 for first 10 1 for the remaining 3 tell me how to make a drl with this thing'