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 == 1):
    even = 2
    nth = n//2
    x = (nth * (even + n-1))//2
    
    odd = 1
    nt = (n//2) + 1
    y = (nt * (odd + n))//2
    
    print(x - y)
else:
    even = 2
    nth = n//2
    x = (nth * (even + n))//2
    
    odd = 1
    nt = n//2
    y = (nt * (odd + n - 1))//2
    
    print(x - y)