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 recursSumIt(n):
	result = 0
	m = n
	if n >= 10:
		result = 5 * int(n / 10)
		n = n % 10
		if n > 10:
			result += recursSumIt(n)
		for i in range(m - (m % 10) + 1, m + 1):
			result += (-i) if i & 1 == 1 else i
	else:
		for i in range(n + 1):
			result += (-i) if i & 1 == 1 else i	
	return result

x = int(input())
print(recursSumIt(x))