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.math.*;

public class Solution {

	public static void main(String[] args) {

		Scanner scn = new Scanner(System.in);
		BigInteger n = new BigInteger(scn.next());
		BigInteger two = new BigInteger("2");
		BigInteger total = new BigInteger("0");
		BigInteger one = new BigInteger("1");
		BigInteger negOne = new BigInteger("-1");
		BigInteger zero = new BigInteger("0");
		if (n.mod(two).equals(zero)) {
			total = n.divide(two);
		} else {
			total = (negOne).multiply(((n.divide(two).add(one))));
		}
		System.out.print(total);
	}
}