Invoice list with negative net value

Dear Team,

I am trying to create the invoice list with two billing types one is normal invoice and second is credit memo.

Invoice with value 100EUR and two credit memo one with net value 10EUR and 150EUR

While creating the invoice list when I am entering the credit memo its taking the negative value

-10 EUR and -150EUR and one invoice with +100EUR

Now the net value of the invoice list is -60EUR.

Could you please suggest why it is taking negative value for both the credit memo.

Regards,

P