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
x =int(input())
allsum=0;
allsum=x*(x+1)//2
n:int =x//2
if(x%2==0):
 evensum=n*(2+x)//2
else:
    evensum = n * (2 + (x-1)) // 2
oodsum=allsum-evensum
sum=evensum-oodsum

print(sum)