Two's complement

If anyone can tell me what this is (excluding Daniel), I'll give them a cookie.

(No googling.)

Isn't it one of them binary number fingy ma bobs?

I know what it is but don't remember how to calculate it. =[ I had to Google it to just remember how to calculate it really.

Edit: Guarantee you 95% of this forum will Google it and act like they know what it was.

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yes i googled it
i would never have known owt about it anyway

I looked at wiki and makes no sense to me

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tell you what, what is?

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-confused-