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 apsum(bg, ed, n):
	return ((bg + ed) * n) / 2

n = int(raw_input())

even = n/2;
odd = n/2 + n%2;

print apsum(2, even * 2, even) - apsum(1, 2 * (odd - 1) + 1, odd)
