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.*;
import java.util.*;

public class CodeForces {
    public static void main(String[] args) {
       Scanner scanner = new Scanner(System.in);
       BigInteger n = new BigInteger(scanner.next());

       double rem = 0;
       if(n.remainder(new BigInteger("2")).doubleValue() != 0) {  //odd
         n = n.add(new BigInteger("1"));
         rem = (Math.pow(-1, n.doubleValue())*n.doubleValue());
       }
       BigInteger remS = new BigDecimal(String.valueOf(rem)).toBigInteger();
       BigInteger big = (n.divide(new BigInteger("2")));
       System.out.println(big.subtract(remS));
    }
}