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
# a simple parser for python. use get_number() and get_word() to read
def main():
    def parser():
        while 1:
            data = list(input().split(' '))
            for number in data:
                if len(number) > 0:
                    yield(number)

    input_parser = parser()
    gets = lambda: next(input_parser)

    def getNum():
        data = gets()
        try:
            return int(data)
        except ValueError:
            return float(data)
    # ---------input---------
    n = getNum()
    print(n//2 - ( n if (n&1)>0 else 0 ))
    

if __name__ == "__main__":
    main()