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())
if(n%2==0):
    x=int(n/2)
    z=x*(x+1)
    o=n-1
    y=int((o//2)+1)
    m=y*y
    print(z-m)
elif(n%2==1):
    j=int((n//2)+1)
    k=j*j
    h=n-1
    l=int(h/2)
    q=l*(l+1)
    print(q-k)
