Calculating Function
题目描述
A. Calculating Function
time limit per test
1 secondmemory limit per test
256 megabytesinput
standard inputoutput
standard outputFor a positive integer n let's define a function f:
f(n)=-1+2-3+..+(-1)nn
Your task is to calculate f(n) for a given integer n.
Input
The single line contains the positive integer n (1≤n≤1015).
Output
Print f(n) in a single line.
Examples
Input
4
Output
2
Input
5
Output
-3
Note
f(4)=-1+2-3+4=2
f(5)=-1+2-3+4-5=-3
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