Teaser 2504: [Hidden crosses]

From The Sunday Times, 19th September 2010 [link] [link]

Here are 18 black cards and there is a cross on the back of some of them.

The clue in any gap indicates how many crosses there are on cards that are adjacent to that gap.

How many black cards have a cross on them?

This puzzle was originally published with no title.

[teaser2504]