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
# import sys
# sys.stdin=open('input.in','r')
# sys.stdout=open('output.out','w')
s=int(input())
n1=int(s/2)
n2=s-n1
ev=n1*(4+(n1-1)*2)
od=-n2*(2+(n2-1)*2)
sum=(ev+od)/2
print(int(sum))



	



















