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
from math import ceil
def fun(num,a,d):
    n=ceil(num/2)
    v=((((2*a)+((n-1)*d))* n)//2)
    return v
    
n=int(input())
su=0
if n%2==0:
    vo=fun(n-1,-1,-2)
    ve=fun(n,2,2)
    su=vo+ve
else:
    vo=fun(n,-1,-2)
    ve=fun(n-1,2,2)
    su=vo+ve
print(su)
