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
//int[] s = Arrays.stream(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();

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

public class Solution {

    public static void main(String[] args) throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        double n = Double.parseDouble(br.readLine());
        double res =0;
        if(n %2 ==0){
            res = (n/2);
        }
        else{
            res = -(n+1)/2; 
        }
        System.out.println(Math.round(res));
        
    }
}
