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
from sys import stdin
debug = False

try:
    import os
    if os.environ['COMPUTERNAME'] == 'KRUEGER-PC':
        debug = True
except KeyError:
    pass


def debug_print(*foo):
    if debug:
        print(foo)

debug_print("debug active")


def print_array(res, size):
    if size:
        print(len(res))
    print(" ".join(map(str, res)))


def input_int_tuple():
    return tuple([x for x in map(int, stdin.readline().split())])


def input_int_array(size=True):
    if size:
        # read
        values = stdin.readline().split()
        assert(len(values) == 1)
    return [x for x in map(int, stdin.readline().split())]


def input_string():
    return stdin.readline().strip()


def count(x):
    return len(bin(x)) - 2


def problem():
  n,  = input_int_tuple()
  if  n % 2 == 0:
    return n //2
  else:
    return n//2 - n

print problem()


#print_array(problem(), False)

#xx = input_string()
