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
def arithmetic(n):
    if n%2!=0:
        even=n//2
        odd=even +1
    else:
        odd=even=n//2
    neg_sum=odd*(-2 + (odd-1)*(-2))//2
    pos_sum=even*(4+ (even-1)*2)//2
   
    return neg_sum+pos_sum



if __name__=='__main__':
    n=int(input())
    print(arithmetic(n))
