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

import sys, math
def rs():
    return sys.stdin.readline().strip()
def ri():
    return int(sys.stdin.readline().strip())
def ras():
    return list(sys.stdin.readline().strip())
def rai():
    return map(int,sys.stdin.readline().strip().split())
def raf():
    return map(float,sys.stdin.readline().strip().split())


def solve():
  n = ri()
  if n%2 == 0:
    return n/2
  else:
    return n /2 - n




print solve()
