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 sys
#sys.stdin = open('input.txt', 'r')
#sys.stdout = open('output.txt', 'w')
def x(y):
	return ((2 + y) * (y // 2) // 2)
def a(y):
	return (1 + y) * ((y + 1) // 2) // 2
n = int(input())
if n % 2 == 0:
	print(x(n) - a(n - 1))
else:	
	print(-a(n) + x(n - 1))