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
# your code goes here
x=int(input())
if x%2==0:
	p=x//2
	q=x//2
else:
	p=x//2
	q=x//2+1
#print(p,q)
m=q*q
n=p*(p+1)
#print(m,n)
print(n-m)