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())
def f(a):
    if a % 2 == 0:
        s1 = ((2 + a) // 2) * ((a - 2) // 2 + 1)
        s2 = ((1 + a -1) // 2) * (a // 2)
        s1 -= s2
        return(s1)
    else:
        s1 = ((2 + a - 1) // 2) * (a // 2)
        s2 = ((1 + a) // 2) * ((a + 2) // 2)
        s1 -= s2
        return(s1)
print(f(n))
        