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
n=int(input())
m=n
s=0
flag=0
if(n%2==0):
    n-=1
    flag=1
temp=n-1
c1=temp//2
if(c1%2==0):
    s+=(-(n//2+1))
else:
    s+=(-1-n)
    s+=(n//2+1)
if(flag==1):
    s+=(m)
print(s)