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
'''
Created on Nov 13, 2014

@author: nkyrt
'''

def solve_it(n):
    e = n if n % 2 == 0 else n-1
    o = n if n % 2 == 1 else n-1
    return (2 + e)*e/4 - (1 + o)*((o+1)/2)/2
    
if __name__ == '__main__':
    print solve_it(int(raw_input()))