Virtual contest is a way to take part in past contest, as close as possible to participation on time. It is supported only ACM-ICPC mode for virtual contests.
If you've seen these problems, a virtual contest is not for you - solve these problems in the archive.
If you just want to solve some problem from a contest, a virtual contest is not for you - solve this problem in the archive.
Never use someone else's code, read the tutorials or communicate with other person during a virtual contest.

No tag edit access

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)^{n}*n*

Your task is to calculate *f*(*n*) for a given integer *n*.

Input

The single line contains the positive integer *n* (1 ≤ *n* ≤ 10^{15}).

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

Codeforces (c) Copyright 2010-2017 Mike Mirzayanov

The only programming contests Web 2.0 platform

Server time: Mar/24/2017 09:16:00 (p1).

Desktop version, switch to mobile version.
User lists

Name |
---|