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())
# s = -1
# res = 0
# for i in range(1,n+1):
#      res = s*i +res
#     #  print(res,i,s)
#      s = s*-1
if n % 2 ==0 :
    print(n // 2 )
else :
    print(-1 * (n+1)//2)
# print(res)