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
ti = lambda : stdin.readline().strip()
ma = lambda fxn, ti : map(fxn, ti.split())
ol = lambda arr : stdout.write(' '.join(str(i) for i in arr) + '\n')
os = lambda i : stdout.write(str(i) + '\n')
olws = lambda arr : stdout.write(''.join(str(i) for i in arr) + '\n')


n = int(ti())
pos = 0
neg = 0
if n % 2 == 0:
	m = n/2
	pos = (2*2*m + m*(m-1)*2) / 2
	neg = -1 * ((2*1*m + m*(m-1)*2)/2)
else:
	m = n/2
	pos = (2*m+m*(m-1))
	m += 1
	neg = -1 * (m + m*(m-1))
os(pos+neg)