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
//package codeforces.R277D2.CalculatingFunction;

import java.io.*;
import java.util.*;

public class CalculatingFunction {

    public static void main(String[] args) throws IOException {

        boolean fileInOut = CalculatingFunction.class.getPackage() != null;

        Scanner sc = new Scanner(new BufferedReader(new InputStreamReader(fileInOut ? CalculatingFunction.class.getResourceAsStream("in.txt") : System.in)));
        Solution.out = new PrintWriter(new BufferedOutputStream(fileInOut ? new FileOutputStream("out.txt") : System.out), true);

        int testCase = fileInOut ? sc.nextInt() : 1;

        for (int i = 0; i < testCase; i++) {
            new Solution().solve(sc.nextLong());
        }

        if (fileInOut) {

            verify(CalculatingFunction.class.getResource("ans.txt").getFile());
        }
    }

    public static void verify(String ansFile) throws IOException {

        String outputFile = "out.txt";

        BufferedReader reader1 = new BufferedReader(new FileReader(ansFile));
        BufferedReader reader2 = new BufferedReader(new FileReader(outputFile));

        String line1 = reader1.readLine();
        String line2 = reader2.readLine();

        boolean areEqual = true;
        int lineNum = 1;

        while (line1 != null || line2 != null) {

            if (line1 == null || line2 == null) {

                areEqual = false;
                break;
            } else if (!line1.equals(line2)) {

                areEqual = false;
                break;
            }

            line1 = reader1.readLine();
            line2 = reader2.readLine();

            lineNum++;
        }

        if (areEqual) {

            System.out.println("All Test Cases Passed !");
        } else {

            System.out.println("Output differ at line " + lineNum);
            System.out.println("ans.txt has " + line1 + " and out.txt has " + line2 + " at line " + lineNum);
        }

        reader1.close();
        reader2.close();
    }
}

class Solution {

    public static PrintWriter out;

    public void solve(long n) {

        if (n % 2 == 0) {
            out.println( n / 2);
            return;
        }

        out.println(-(n+1)/2);
    }
}
