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())
# a,b,c,d=input().split()
# a=int(a)
# b=int(b)
# c=int(c)
# d=int(d)
# e,f,g=input().split()
# e=int(e)
# f=int(f)
# g=int(g)
# mine=a[0]
# for i in range(1,n):
#     for j in range(mine):
#         print(a[1:])
n = int(input())
if n%2==0:
    print(int(n/2))
else:
    print(int((((n+1)/2)-(n+1))))