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.util.*;
import java.io.*;
import java.math.*;
import java.awt.geom.*;

public class _486A implements Runnable {

    static BufferedReader br = null;
    static PrintWriter pw = null;
    static StringTokenizer stk = new StringTokenizer("");

    public static void main(String[] args) {
        new Thread(new _486A()).run();
    }

    public void run() {
         
             br = new BufferedReader(new InputStreamReader(System.in));
            pw = new PrintWriter(new OutputStreamWriter(System.out));
            solver();
            pw.close();
    }
    public void nline() {
        try {
            stk = new StringTokenizer(br.readLine());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    private String nstr() {
        while (!stk.hasMoreTokens())
            nline();
        return stk.nextToken();
    }

    private int ni() {
        return Integer.valueOf(nstr());
    }

    private long nl() {
        return Long.valueOf(nstr());
    }

    private double nd() {
        return Double.valueOf(nstr());
    }

    String nextLine() {
        try {
            return br.readLine();
        } catch (IOException e) {
        }
        return null;
    }


    private void solver() {
        long n = nl();
        System.out.print((n%2==0) ? n/2 : (n/2+1)*(-1));
        
        
    }
}