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
line = raw_input().split()
line = int(line[0])
par = 0
impar = 0
if (line%2==0):
	par = line
	impar = line - 1
else:
	impar = line
	par = line - 1

val_impar = ((impar + 1)/2)*((impar + 1)/2)
val_par = (par/2)
val_par = val_par*(val_par+1)
print val_par - val_impar