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==1):
    print(-1);
else:
    if(n%2==0):
        od=n//2;
        ev=n//2;
    else:
        od=(n//2)+1;
        ev=n//2;
    smo=(od*(-2-((od-1)*2)))//2;
    sme=(ev*(4+((ev-1)*2)))//2;
    sm=smo+sme;
    print(sm);