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
# Observation: All odd numbers are negative, all even are positive
# So sum all odd numbers and sum all even numbers then just subtract to obtain answer
# Sum of odd numbers are calculated with this formula: Given a number n, ensure that number is even, if not, add one to it to make it even. Next, n//2 and then square that number. That will be the answer

# Sum of even numbers formula:
# Suppose 1+2+3+...+n where n is the last EVEN number in the series. So if n is not even, then just n-1
# magic_number = (first_even_number + last_even_number)/2 -1
# magic_number = (2+100)/2 -1 = 50     where n is 100
# Now, you take:
# magic_number*(magic_number+1) = your sum


def prob486a(x):
    # Pre-processing. variable n is for odd calculations, k,j are for even calculations
    if x % 2 == 0:
        n = x
        k = x
    else:
        n = x + 1
        k = x - 1
    # Calculate sum of odd numbers
    sum_odd_num = (n // 2)**2

    # Calculate sum of even numbers
    if x >= 2:
        magic_number = ((2 + k) // 2) - 1
        sum_even_num = magic_number * (magic_number + 1)
    else:
        sum_even_num = 0
    return sum_even_num - sum_odd_num


if __name__ == "__main__":
    x = int(input())
    print(prob486a(x))
