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
l=int(raw_input())
##j=list()
##for i in range(1,l/2+1):
##    j.append(4*i)
##sum1=sum(range(1,l+1))
##sum2=sum(range(1,(l/2)+1))
sum1=(l*(l+1))/2
sum2=((l/2)*((l/2)+1))/2
print(4*sum2-sum1)