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Pyth - 15 10 * .5 = 5

smr-QdhQUQ

Try it online.

Expects input on stdin. Independently discovered algorithm. Thanks @Sp3000 for helping me stick the last Q in there :P Also, irony? XD

Explanation:

Q=eval(input())       : implicit
s                     : The sum of...
 m      UQ            : map(...,range(Q))
  r-QdhQ              : range(Q-d,Q+1)