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
x = int(input())

'''
new_num = 0
for i in range(x):
	i += 1
	new_num += (-1)**(i) * (i)

print(new_num)
'''

if x % 2 == 0:
	print(round(x/2))
else:
	print(round(-((x+1)/2)))
