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
#!/usr/bin/python
from math import floor

'''
Problem 486A: http://codeforces.com/problemset/problem/486/A
Solved on: 2015-02-08
Result: Accepted 61 ms 8 KB
'''

def main():
	n=int(raw_input())
	print n/2 if n%2==0 else int(floor(n/2)-n)

if __name__=='__main__':
	main()