Before answering your question directly, just how important are these slides and negatives? Are they the sort of stuff that would be 'life changing' if you lost them? If so, I would not consider any form of CD or DVD as backup media. When first produced they were considered to be an indestructible form of data storage, but it has become clear over time that they ain't quite so good as first thought.
I would be looking at doubling up on backup, once to an external hard disk and again to some form of on-line storage. If you lose one, you've still got the other.
If you don't want to go that far and want to stick with DVD or CD then always do two copies of everything, just in case.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.