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())
chet = n//2
nechet = n//2 + n%2
a1 = 1
a2 = 2
if n%2 == 1:
    an1 = n
    an2 = n-1
else:
    an1 = n-1
    an2 = n
s1 = (a1+an1)*nechet//2
s2 = (a2+an2)*chet//2
print(s2-s1)