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
#var=list(reversed(sorted(B))) for sorting form b to s // output += [item] # see below note // a = [int(x) for x in input().split()]
#you can insert two parameter in one element in a list ex. a.append([a,b]) // sample() return asub of input that it catch
n= int(input())
if n%2==0:
    print(n//2)
else:
    print(-((n//2)+1))