For 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
import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {
       
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        long n = Long.parseLong(s.nextLine());
        long sEven, sOdd, sum;
        long nEven, nOdd;
        if ((n % 2) == 1)   nOdd = (n + 1) / 2;
        else    nOdd = n / 2;
        nEven  = n / 2;
        
        //System.out.println(nEven);
        //System.out.println(nOdd);
        
        sEven = nEven * (nEven + 1);
        //sEven = n * (n + 1);
        sOdd = (nOdd * nOdd);// + 2 * nOdd;
        //sOdd = n * (n + 2);
        
        //System.out.println(sEven);
        //System.out.println(sOdd);
        
        System.out.println(sEven - sOdd);
    }
}