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
# input: me dan n , tengo que calcular f(n)= -1+2-3....+(-1)^n(n)
# output: devuelvo f(n) (ej, f(4)= -1+2-3+4 = 2)

n=int(input())
x=1
#i=0
#while x <=n :
#    i+= ((-1)**x)*x
#    x+=1
if n%2==0:
    print(int(n/2))
else:
    print(int(-((n+1)/2)))