CJam, 18 bytes
T2/(f.-~(+.*:-z.5*
[Try it online.][1]
[1]: http://cjam.aditsu.net/#code=q~%3AT%3B%20e%23%20Store%20input%20in%20T.%0A%0AT2%2F(f.-~(%2B.%3A-z.5&input=%5B1%202%204%202%203%207%5D
Dennis
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