What is the formula for standard deviation?
Have you tried either (1) an elementary textbook on Statistics, or (2) WikiPedia? In fact, check into this: http://en.wikipedia.org/wiki/Standard_deviation
What seems unusual, is "standard" deviation exists and is distinct from "average absolute value of the deviations".
You might also want to decide whether you are interested in the standard deviation of a probality distribution or of a sample from a probability distribution. The formulas are slightly different.
This is a pretty good webpage:
Calculating Standard Deviation
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