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

public class Main{
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        BigInteger n = new BigInteger(scanner.next());
        System.out.println(n.remainder(BigInteger.valueOf(2)).compareTo(BigInteger.valueOf(0)) == 0 ?
            n.divide(BigInteger.valueOf(2)) :
            n.subtract(BigInteger.valueOf(1)).divide(BigInteger.valueOf(2)).subtract(n));
    }
}
