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')


t = int(input())

s  = 0
if t%2==0:
	print(t//2)
else:
	t = -1*(t//2) - 1
	print(t)