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 math
from fractions import Fraction as frac

MOD = 1e9 + 7

def gcd(a, b):
    if b == 0:
        return a
    return gcd(b, a % b)

def lcm(a, b):
    return a * b // gcd(a, b)

def no():
    print("NO")
    return

def yes():
    print("YES")
    return

def solve(case_no):
    n = int(input())
    ans = 0
    if n % 2 == 1:
        ans = -n
        n -= 1
    ans += n // 2
    print(ans)


t = 1
# t = int(input())
for i in range(1, t + 1):
    solve(i)
