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 calc(n):
    if n == 1:
        return -1
    elif n%2 == 0:
        res = n + calc(n-1)
    else:
        res = -n + calc(n-1)
    return res

x = int(raw_input())
# x could not be larger than 995.

print calc(x)
'''

'''
y = int(raw_input())

summ = 0
for i in range(1, y+1):
    if i%2 == 0:
        summ += i
    else:
        summ -= i

print summ
'''

z = int(raw_input())

if z%2 == 0:
    print z/2
else:
    print -(z+1)/2
