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 retrieve_data():
    func = int(raw_input())
    return func

def function(data):
    if data % 2 == 0:
        return data / 2
    else:
        return int(-1 * (data/2 + 1))                   

def main():
    data = retrieve_data()
    print function(data)

if __name__ == '__main__':
    main()
