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 = int(input())
# f(n) = -1 + 2 + -3 + ... + (-1)^n * n

sum = 0
'''
for i in range(n+1):
    sum += ((-1) ** i) * i
'''

if n % 2 == 0:
    sum = n // 2
else:
    sum = 0 - (n + 1)//2

print(sum)