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
m=input()
if(m%2!=0) :
    temp=m
    m=m-1
    n=(m*(m+1))/2
    s=((m/2)*(m/2+1))
    print(s-(n-s)-temp)
else :
    n=(m*(m+1))/2
    s=((m/2)*(m/2+1))
    n=(m*(m+1))/2
    s=((m/2)*(m/2+1))
    print(s-(n-s))
