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
from sys import stdin, stdout
import sys
import math
def get_int(): return int(stdin.readline().strip())
def get_ints(): return map(int,stdin.readline().strip().split()) 
def get_array(): return list(map(int,stdin.readline().strip().split()))
def get_string(): return stdin.readline().strip()
def op(c): return stdout.write(c)
#from collections import defaultdict 
#for _ in range(int(stdin.readline())):
n=get_int()
if n%2==0:
    print(n//2)
else:
    print(-(n+1)//2)