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
def fun():
    n=int(input())
    odd=0
    even=0
    if n%2==0:
        odd=n//2
        even=n-odd
    else:
        odd=(n//2)+1
        even=n-odd
    sumofodd=(odd*((2*1)+((odd-1)*2)))//2
    sumofeven=(even*((2*2)+((even-1)*2)))//2
    print(int(sumofeven-sumofodd))
fun();