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())
a = 0
b = 0
for i in range(n):
    if a == 0:
        b = -1
    elif a % 2 == 0 and a != 0:
        b -= i
    else:
        b += i

    a += 1

if b > 0:
    print(b + 1)
else:
    print(b)
'''
n=int(input());print(n//2-n%2*n)

