When you level up, do you spend the xp needed, lowering your total by the amount spent, or do you just keep...

When you level up, do you spend the xp needed, lowering your total by the amount spent, or do you just keep accumulating the same number against the level-chart?

I've assumed the former, and I think a progression where first and second level cost the same but everything else costs increasing amounts is weird -- so that supports my assumption. But not everyone in my group made the same assumption. And I think the strictest interpretation of the rules as written goes against my assumption.

So, thoughts?