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
#coding=utf-8

# if __name__ == "__main__":
#     # 读取第一行的n
#     n = int(sys.stdin.readline().strip())
#     for i in range(n):
#         # 读取每一行
#         line = sys.stdin.readline().strip()
#         # 把每一行的数字分隔后转化成int列表
#         values = map(int, line.split())

# n = int(input())
# for i in range(n):
#     line = input().strip()
#     values = list(map(int, line.split()))


def Q24(n):
    if n % 2 == 0:
        res = n // 2
    elif n % 2 != 0:
        res = (n - 1)//2 - n
    return res



if __name__ == "__main__":
    line = int(input())
    print(Q24(line))








