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
'''                             Hariom_Pandey
                                 15.11.2019                                        '''
import math as mt
import sys as sy
class abc:
    def mkl(self,):
        n=int(input())
        
        if(n%2==0):
            print(n//2)
        else:
    	    print('-'+str(int(n//2)+1))

b=abc()
b.mkl()