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 num():
    n = int(input())
    l1 = 0
    l2 = 0
    if(n%2 == 0):
        l1 = n
        l2 = n-1
    
    else:
        l1 = n-1
        l2 = n
    c1 = (l1-2)//2+1
    c2 = (l2-1)//2+1
    s1 = c1*(2+l1)//2
    s2 = c2*(1+l2)//2
    print((int)(s1-s2))
num()