Sales BOM --> Free rounding unit maintainence

Hi

in BOM , if for 10 "X"materials --> 1 'Y" material is free

In order, if i enter 20 materials "X" --> 2 " Y" materials are coming,

Need:

if i enter 25 materials . it is taking 2.5. However  i want it should be 2 .

system should take rounded value as 2.

Pls assist

Sachin