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.*;

public class App {
    
    public static void main (String[] arg) {
        
        Scanner sc = new Scanner(System.in);
        
        long n = sc.nextLong();
        
        long zaporna, kladna;
        
        long tmp;
        
        if ( n % 2 != 0 ) {
            
            System.out.println((1+n)/(-2));
            return;
            
        } else {

            System.out.println(n/2); 
            return;
        }
        
    }
    
    public static long function ( long n ) {
        
        if ( n == 1 ) return -1;
        return (long) Math.pow(-1, n) * n + function(n - 1);
    }
    
    public static long calculate ( long m, long n, long a ) {
        
        long result;
        
        if ( m % a == 0 ) result = m / a;
        else result = m / a + 1;
        
        if ( n % a == 0 ) result *= n / a;
        else result = result * ( n / a + 1 );
        
        return result;
    }
    
    
}