Question: Is there a single word or compact term for a bet where you are **guaranteed** to get your input back?

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> Say you bet $1 on a dice roll and get $1 for each eye on the dice roll. Since the dice has a minimum of one eye, you are getting your input of $1 back no matter what.
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This of course is a very bad bet for the bank.

But how is that called?

Answer: 
[A sure thing](http://www.thefreedictionary.com/sure+thing):

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> An outcome that is certain or assured; a certainty; something guaranteed to be successful; something that is or should be a certain success, as a bet. - TFD
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> > The Picasso Signature Is Still a Sure Thing - NYT
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Also, a *foregone conclusion*:

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> An end or a result regarded as inevitable:
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I'm not sure that even qualifies as bet.

A bet usually implies the risk of losing the initial money, but in this case the risk is just not there.