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
# http://codeforces.com/problemset/problem/486/A

n = input()
"""
DUMBEST SOLUTION
value = 0
for i in range(1,n+1):
    if i%2 == 0:
        value = value + i
    else:
        value = value - i
"""

if n % 2 == 0:
    n = n/2
    print n
else:
    n = n/2
    print -(n+1)