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
n = int(input())

# if n is even, half of n are odd numbers and other half even numbers
# if n is odd, (n//2)+1 are odd numbers and n//2 even numbers

# sum of n odd numbers = n**2
# sum of n even numbers = n*(n+1)
nby2 = n//2
if n % 2 == 1:  # odd
    sum_of_evens = (nby2)*((nby2)+1)
    sum_of_odds = ((nby2)+1)**2
elif n % 2 == 0:  # even
    sum_of_evens = (nby2)*((nby2)+1)
    sum_of_odds = ((nby2))**2

print(sum_of_evens-sum_of_odds)
