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
from math import ceil

X = int(input())
print((X * (X + 1)) // 2 - (2 * ceil(X / 2) ** 2))

# UB_CodeForces
# Advice: Don't limit your challenge, challenge your limits
# Location: Behind my desk next my quince lemon tea
# Caption: It's beautiful day and I what to make it perfect
# CodeNumber: 451
