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.Arrays;
import java.util.Scanner;


public class Cf{
    static Scanner scan=new Scanner(System.in);
    public static void main(String[] args) {
        int t=1;
//         t=scan.nextInt();
        while(t-->0){
            solve();
        }
    }
    static void  solve(){
        long N=scan.nextLong();
        if(N%2==0){
            Long half=N/2;
            System.out.println(half);

        }else{
            long half=(N-1)/2-N;
            System.out.println(half);
        }

    }
    }



