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
# Tasks
# - Create a script to `WORDLIST` problem using the .in and .out files
# - IMPLEMENT ROT13, ROT47, ETC...
# - Implement combinations(arr, r) -> returns all possible combinations or a generator
# --------------------
# 158 B - Taxi
# n = int(input())
# A = list(map(int, input().split()))
# A.sort(reverse=True)
# count = 0
# # write your greedy code here...
# print(count)
# --------------------


# def factors(n):
#     for i in range(1, n // 2 + 1):
#         if n % i == 0:
#             yield i
#     yield n


# n, x = map(int, input().split())
# count = 0
# for i in factors(x):
#     if i <= n and x // i <= n:
#         count += 1
# print(count)

n = int(input())
if n % 2 == 0:
    print(n // 2)
else:
    print(-(n + 1) // 2)
