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 = raw_input()
n = int(n)

def f(n):
    n_c = n/2*2
    s1 = 0
    s2 = 0
    if ( n == 1 ): return -1
    if ( n == 2 ): return 1
    if ( n%2 != 0 ):
        nn = n
        nc = n-1
        s1 = (1+nn)/2*(nc/2+1)
        s2 = (2+nc)/2*(nc/2)
    else:
        nn = n-1
        nc = n
        s1 = (1+nn)/2*(nc/2)
        s2 = (2+nc)/2*(nc/2)
    return s2-s1

print f(n)
