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
#Calculating_Function.py
import sys,math
num = int(sys.stdin.readline())
count_odd = count_even = 0
if(num%2==1):
    ans = int((-1)*((num+1)/2))
else:
    ans = int((num/2))
print(ans)