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 static java.lang.System.*;
import static java.lang.Math.*;
import java.io.*;
import java.util.*;
import java.math.*;

public class Alyona {
    public static void main(String[] args) throws IOException{
        Scanner input=new Scanner(System.in);
        BigInteger n=new BigInteger(input.nextLine());
        if(n.mod(BigInteger.valueOf(2)).compareTo(BigInteger.ZERO)!=0){
            out.println((n.add(BigInteger.ONE)).divide(BigInteger.valueOf(2)).multiply(BigInteger.valueOf(-1)));
        }
        else{
            out.println(n.divide(BigInteger.valueOf(2)));
        }
    }
}
