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
n1=int(input())
e=(n1//2)
if n1%2:
    o=(n1//2)+1
else:
    o=n1//2
print((e*(e+1))-(o**2))
# -1+2-3+4.....((-1)**n)*n 
# sum of n even numbers is 'n*(n+1)'
# sum of n odd numbers is 'n^2'