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())
if(n%2==0):
    e = n
    o = n-1
else:
    e = n-1
    o = n
e = int(e/2)
sum_e = e*(e+1)
o = int(o/2)
sum_o = o+1 + o*(o+1)
print(sum_e-sum_o)