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 f(n):
    # sum of even_numbers
    ## first of all we need to determine n_even, a_1, a_n
    n_even, a_1, a_n = n//2, 2, n if n%2==0 else n-1
    even_sum = n_even*((a_1+a_n)//2)
    
    # similarly let's get the sum of odd numbers
    n_odd, a_1, a_n  = n-n//2, 1, n if n%2!=0 else n-1
    odd_sum  = n_odd*((a_1+a_n)//2)
    
    print(even_sum - odd_sum)
    
f(int(input()))