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 main():
    n=int(input())
    if n%2==0:
        k=(n-2)//2+1
        c=(k*(2+n))//2
        d=(k*(1+n-1))//2
    else:
        k=(n-1-2)//2+1
        c=(k*(2+n-1))//2
        d=((k+1)*(1+n))//2
    #print(k,c,d)
    print(c-d)
main()