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())
x=0
y=0
z=0
a=0
b=0
c=0
b=n/2
a=int(b*(b+1))
x=int(b*b)

if n%2==0:
    print (a-x)
else:
    print (a-x-n)
