CF486A Calculating Function

For a positive integer $ n $ let's define a function $ f $ : $ f(n)=-1+2-3+..+(-1)^{n}n $ Your task is to calculate $ f(n) $ for a given integer $ n $ .

The single line contains the positive integer $ n $ ( $ 1

Print $ f(n) $ in a single line.

$ f(4)=-1+2-3+4=2 $ $ f(5)=-1+2-3+4-5=-3 $