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
#n, a = input().split()

a = input()
#k = input().split()
#k = input()
#n = input()
#k = [i for i in k]
#k = [input().split() for i in range(int(a))]
#n = input().split()

#n = [i for i in n[0]]

'''
cont = [
'a', 'b', 'c', 'd', 'e', 'f', 'g',
'h', 'i', 'j', 'k', 'l', 'm', 'n',
'o', 'p', 'q', 'r', 's', 't', 'u',
'v', 'w', 'x', 'y', 'z'
]
'''
#n = a[1]
#a = a[0]
a = int(a)
if a % 2 == 1:
    s = ((a // 2) + 1) * -1
else:
    s = a // 2
print(s)