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

# def gcd(lft, rght):
#     if lft < rght:
#         rght, lft = lft, rght
#     if rght == 0:
#         return lft
#     return gcd(rght, lft % rght)
#
#
# def lcm(lft, rght):
#     return (lft * rght) / gcd(lft, rght)


lookup_pow = int(input())

print(
    int(lookup_pow / 2) * (-1 if lookup_pow % 2 else 1) - (1 if lookup_pow % 2 else 0)
)
