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
'''
# Time limit exceeded bas el code sa7
n = int(input())
sum = 0
for i in range(n):
    x = pow(-1, (i + 1)) * (i + 1)
    sum = sum + x
print(sum)
'''
n = int(input())
if n % 2 == 0:
    sum = n / 2
else:
    sum = (n-1)/2 - n
print(int(sum))