Format a cell to always be negative

It seems Excel can do anything we want it to. But, can you format a cell so that when a number is entered into it by someone that it will always be negative? I tried adding a comment to the cell to warn users to enter it as negative but if the user is using the tab or enter button the mouse focus doesn't land on the cell and the comment doesn't pop up. I need to make sure that number is always negative.

Thanks in advance for all your help and suggestions.