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:
    n=int(n)/2
else:
    if n!=1:
        n=(n/2)-n
        n1=int(n)
        n2=float(n)
        if n1%n2!=0:
            n=int(n)-1
    else:
        n=(n/2)-n-1
    
    
print(int(n))
