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
n=int(input())
# func=0
# for i in range(1,n+1):
# 	func+=i*((-1)**i)
# 	#print(func)
# print(func)
if n%2==0:
	a=n//2
	ans=-a*(1+(a-1)*2)+a*(2+(a-1)*2)
	print(ans)
else:
	a=n//2
	ans=-a*(1+(a-1)*2)+a*(2+(a-1)*2)-n
	print(ans)	
