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
n=int(input())
result=n/2
if n%2==1:
    result= -(result+0.5)
print(int(result))



#n=int(input())
#result=0
#for i in range(n):
#    result+=((-1)**(i+1))*(i+1)
#print(result)

#f(5) =  - 1 + 2 - 3 + 4 - 5 =  - 3
#adad dakhel f ro mide result ro mikhad
