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

in_num = input()
ou_sum = 0
#for i in xrange(1, in_num + 1):
if(in_num %2 == 0):
	ou_sum = in_num / 2
else:
	ou_sum = - (in_num + 1) / 2
print ou_sum

