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
from sys import stdin, stdout

def fun():
    n = stdin.readline()
    if int(n)%2 == 0:
        sum = int(int(n)/2)
    else:
        sum = int(-1*((int(n)+1)/2))
        
    stdout.write(str(sum))

if __name__ == "__main__":
    fun()
   			  		  	 	 	 		 	  	   	 	