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
import sys

local_launch = False
try:
    open('LOCAL').read()
    local_launch = True
except:
    pass
input_data = open('INPUT.txt').read() if local_launch else sys.stdin.read()
done = {'i': 0}
tokens = input_data.split()

def next_token():
    result = tokens[done['i']]
    done['i'] += 1
    return result

def next_int():
    return int(next_token())

def next_str():
    return next_token()

############## BEGIN OF CODE #####################
n = next_int()
if n%2 == 0:
    print(n//2)
else:
    print(-(n//2)-1)

















# на вход число
# сказать хорошее или плохое число


############# END OF CODE #####################