It depends. 

If this amount is less than your basis, enter it as a negative amount. 

if this amount is more than your basis, enter zero. 

Basis is your cost in the investment. It includes the purchase price of the initial investment (plus commissions and other purchase expenses), plus or minus any basis adjustments that occur while you own your interest in the investment.

Your basis may equal your initial investment, but often it does not. You can't deduct a loss on an investment that is greater than your tax "basis".