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
num = int(input())
odd = 0
even = 0
def Sum(a,step):
    sum = (step*((2*a)+((step-1))*2))
    return int(sum//2)

if(num%2==0):
    odd = Sum(1,(num//2))
    even = Sum(2,(num//2))
else:
    even = Sum(2,(num//2))
    odd = Sum(1,(num//2)+1)
print(even - odd)