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
#q = int(input())

def f(l,r):
    ans=0
    n = abs(r)-abs(l)+1
    if l%2!=0 and l>0:
        l=-l
    if r%2!=0 and r>0:
        r=-r
    if n==1:
        return r
    if n%2!=0:
        return (l+r)/2
    else:
        if l<0 and r>0:
            return l+f(-(l)+1,r)
        elif l>0 and r<0:
            return l+f(-(l)-1,r)

for i in range(1):
    nn = int(input())
    print(int(f(1,nn)))