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.math.BigInteger;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        
        BigInteger n = BigInteger.valueOf(in.nextLong());
        BigInteger odd = ( n.remainder(BigInteger.valueOf(2)).equals(BigInteger.valueOf(0)) )? 
                         ( n.divide(BigInteger.valueOf(2)).multiply(n.divide(BigInteger.valueOf(2))) ) 
                       : ( n.add(BigInteger.valueOf(1)).divide(BigInteger.valueOf(2)).multiply(n.add(BigInteger.valueOf(1)).divide(BigInteger.valueOf(2))) ) ;
        
        BigInteger even = (n.remainder(BigInteger.valueOf(2)).equals(BigInteger.valueOf(0)))? 
                          ( n.multiply(n.add(BigInteger.valueOf(2))).divide(BigInteger.valueOf(4)) ) 
                        : ( n.subtract(BigInteger.valueOf(1)).multiply(n.add(BigInteger.valueOf(1))).divide(BigInteger.valueOf(4)) ) ;
        
  
        System.out.println(even.subtract(odd) );
        
    }
}