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
/******************************************************************************

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import java.util.*;
import java.io.*;
import java.lang.*;
public class Main
{
    public static long fun(long n)
    {
        if(n%2==0)
        return n/2;
        else
        return -1*(n/2+1);
    }
	public static void main(String[] args) {
		//System.out.println("Hello World");
		Scanner s=new Scanner(System.in);
		long n=s.nextLong();
		long ans=fun(n);
		System.out.println(ans);
	}
}
