Serial Numbers in WM

Hi Guys

I have some materials that have serial numbers and I am using continuous Inventory in WM

When I carry out a count and I have a gain or a loss How do I enter the serial numbers?

When I try to do transaction LI21 to clear the differences in IM I get the message

Maintain serial numbers for total quantity

How do I specify the serial numbers that make up the difference

Cheers Frenchy