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
a = int(raw_input())

# if a == 1:
# 	sum_even = 0
# 	sum_odd = -1 
# elif a==2:
# 	sum_even = 2
# 	sum_odd = -1
# elif a==3:
# 	sum_odd = -4
# 	sum_even = 2
# elif a%2: #odd
# 	sum_even = a/4 * (2 + (a/2 -1))  
# 	sum_odd = (a/2 + 1)/2 * (-1 + -(a))
# else: #even
# 	sum_even = a/4 * (2 + a)
# 	sum_odd = (a/2 + 1)/2 * (-1 + -(a-1))

if a%2: #odd
	x = a/2 + 1
	print -x
else:
	print a/2
#print sum_odd+sum_even