Free Goods scheme for 3-4 different materials on 1 material

hi

I want to give free goods scheme of 3-4 different material on one material at one time in sales order.

Example : For Material "A" (50 qty) <----- materials B (2 qty) + Material C (1 qty)+ Material D (4 qty) + Material E (1 qty) are free

What needs to be done? pls assist

Regards

Sachin Karale