For a positive integer n let's define a function f:

f(n) =  - 1 + 2 - 3 + .. + ( - 1)^{n}n 

Your task is to calculate f(n) for a given integer n.


-----Input-----

The single line contains the positive integer n (1 ≤ n ≤ 10^15).


-----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

Difficulty: interview

Starter code:

Solutions:
n=int(input())

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


n = int(input())

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

n = int(input())
if(n % 2):
	print(n//2-n)
else:
	print(n//2)

n = int(input())
if (n % 2 == 0):
    print (n // 2)
else:
    print(-n // 2)

n = int(input())
print((n+1)//2*(1,-1)[n%2])


print((lambda x: x // 2 - x * (x % 2 == 1))(int(input())))


n = int(input())
print(-n * (n + 1) // 2 + 2 * (n // 2) * (n // 2 + 1))


n = int(input())

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


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








n = int(input())

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

n = int(input())
f = n // 2
if n % 2 == 1:
	f -= n
print(f)

import sys
#sys.stdin = open('input.txt', 'r')
#sys.stdout = open('output.txt', 'w')
def x(y):
	return ((2 + y) * (y // 2) // 2)
def a(y):
	return (1 + y) * ((y + 1) // 2) // 2
n = int(input())
if n % 2 == 0:
	print(x(n) - a(n - 1))
else:	
	print(-a(n) + x(n - 1))


a=int(input(''))
if a%2 == 0:
    print(a // 2)
else:
    print(-(a+1)//2)


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


import math
n = int(input())
print((-1) ** n * math.ceil(n / 2))


def f(n):
	if n%2 == 0:
		return (n//2)
	return (-n+n//2)
def main():
  n = int(input())
  print(f(n))


def __starting_point():
    main()
__starting_point()

'''
Created on 11-Nov-2014

@author: akash
'''
n = int(input())
e = int(n / 2)
o = int(n / 2)
if n & 1 :
    o += 1
print(e * (e + 1) - o ** 2)
    


n = int(input(''))

arv = n/2

if (n%2==1):
    arv = -1 *(arv + 0.5)

print("%.0f" % arv)



def main():
    n = int(input())
    if n % 2 == 0:
        res = int(n/2)
    else:
        res = int(-n/2-1)
    print(res)

main()


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


__author__ = 'Rakshak.R.Hegde'


def AP(a, n, d):
    return (n * (2 * a + (n - 1) * d)) >> 1


n = int(input())
ans = AP(2, n >> 1, 2) - AP(1, (n + 1) >> 1, 2)
print(ans)

x = input()
x = int(x)
add1 = int(x/2)
add2 = x - add1

add1 = add1*(add1+1)
add2 = add2**2

print (add1 - add2)

n = int(input())
le = n
if n % 2 == 1:
    le -= 1
lo = n
if n % 2 == 0:
    lo -= 1
def f(fir, las):
    fir = int(fir)
    las = int(las)
    return (fir + las)*( (las - fir) // 2 + 1) // 2
print(f(2,le) - f(1,lo))


n = int(input())
if n == 1:
    print(-1)
elif n % 2 == 0:
    print(n // 2)
else:
    print((n + 1) // 2 * (-1))


inp=input().split(" ")
k=0
l=[]
a=int(inp[0])
n=((-1)**a)*a
if n%2==0:
    k=n/2
else:
    k=(n//2)
print(int(k))