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):
    v=n//2;
    a=v*(2+n);
    a//=2;
    b=v*(n);
    b//=2;
    sum=a-b;
else:
    v=(n-1)//2;
    a=v*(n+1);
    a//=2;
    b=(v+1)*(n+1);
    b//=2;
    sum=a-b;
print(int(sum))