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
import math
st=''

def func(n):
    if n%2:
        return((n//2)-n)
    return(n//2)





for _ in range(1):#int(input())):
    #x,y,z=map(int,input().split())
    n = int(input())
    #l1=[]
    #inp=input().split()
    #s=input()
    #l1=list(map(int,input().split()))
    #l2 = list(map(int, input().split()))
    #l1=input().split()
    #l2=input().split()
    st+=str(func(n))+'\n'

print(st)

