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 sys import stdin
def sums(l,r):
    terms=((r-l)//2)+1
    return (terms*(r+l))//2
n=int(stdin.readline())
sum1=0
if n%2==0:
    sum1+=sums(2,n)
    sum1-=sums(1,n-1)
else:
    sum1+=sums(2,n-1)
    sum1-=sums(1,n)
print(sum1)
        
    