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
x=int(input())
def f(x):
    if x%2==0:
        return x/2
    else :
        if x<10**7:
            s=0
            signe=-1
            for i in range(1,x+1):
                s=s+signe*i
                signe=signe*-1
            return s
        else:
            return (((x-1)/2)+1)*-1
print(int(f(x)))