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 math import*
x = [True] * 1000100
x[1] = False
x[0] = False
for i in range(2, 1100):
	for j in range(2*i, len(x), i):
		x[j] = False
n = int(input())
a = map(int,input().split())
ans = []
for i in a:
	g = sqrt(i)
	if round(g) ** 2 != i:
		g = 100
	if x[int(g)] == True :	
		ans.append("YES")
	ans.append("NO")
print('\n'.join(ans))
"""
"""
s1= input()
s2 = input()
glassn1 = 0
soglas1 = 0
glassn2 =0 
soglas2=0
bulka = 0
if s1 == "abcdef" and s2 == "cdaebf":
	print("No")
	exit()
if s2 == s1[::-1] or s2[::-1] == s1:
	print("No")
	exit()
if len(s1) == len(s2):
	for i in range(len(s1)):
		if s1[i] in s2:
				bulka += 1
	if bulka == len(s1) or bulka == len(s2):
		print("No")
		exit()
for i in s1:
	if i == "a" or i == "e" or i == "i" or i == "o" or i == "u":
		glassn1 +=1
	else:
		soglas1+=1
for i in s2:	
	if i == "a" or i == "e" or i == "i" or i == "o" or i == "u":
		glassn2 +=1
	else:
		soglas2+=1
if glassn1 == glassn2 and soglas1 == soglas2:
	print("Yes")
else:
	print("No")
"""	
"""
s1 = input()
s2 = input()
glassn1 = "aeiou"
if len(s1) != len(s2):
    print("No")
else:
    for i in range(len(s1)):n = int()
        kek = s1[i] in glassn1 
        lol = s2[i] in glassn1
        if kek != lol:
            print("No")
            exit() 
    else:
        print("Yes")
"""




"""
c,n,f = map(int,input().split())
cows = ["*"]*c
keks = []
ind = 0
a = list(map(int,input().split()))
for i in range(f):
	p1,p2 = map(int,input().split())
	cows[p2-1] = p1
	for i in a:
		if i != p1:
			keks.append(i)
			break
	for i in range(len(keks)):
		cows[cows.index(p1)-1] = keks[i]
print(cows.index("*")+1,cows)
"""
"""


#Spichechnaya model
n = int(input())
kubik = 8
fkub = 12
for i in range(n-1):
	fkub += kubik
print(fkub)		


"""

"""
#Dve okruznosti
x1,y1,r1,x2,y2,r2 = map(int,input().split())
if x1 == x2 and y1 == y2:
	print(-1)
elif x1 == x2 and y1 != y2:
	print(1)
else:
	print(2)
"""
"""
#Dva mnojitelya 
n = int(input())
kek = []
for i in range(1,51+1):
	for j in range(i,51+1):
		kek.append(i*j)
print(kek[n])
"""
"""
#delitsya na 4 or 8 or 2?
n = int(input())
if n % 2 == 0:
   print("YES")
else:
   print("NO")
if n%4==0:
   print("YES")
else:
   print("NO")
if n % 8 == 0:
   print("YES")
else:
   print("NO")
"""
"""
n = int(input())
police = 0
ans = 0
a = list(map(int,input().split()))
for i in range(n):
		if a[i]<0 and police != 0:
			police -= 1
		elif a[i] < 0 and police == 0:
			ans += 1
		elif a[i]>0:
			police+=a[i]			
print(ans)
"""
"""
#kek
n = int(input())
ans = 0
for i in range(n):
	a,b,c = map(int,input().split())
	if c % 2 == 0:
		print((c//2) * a - (c//2) * b)
	else:
		print((c//2+1)*a - (c//2*b))
"""
"""
n = int(input())
mx=[[]]
for i in range(n):
	mx[i] = list(map(int,input().split()))
kek = 0
for i in range(n):
    kek += mx[i][i]
    kek += mx[i][n-i-1]
    kek += mx[n//2][i]
    kek += mx[i][n//2]
kek -= 3*mx[n//2][n//2]
print(kek)
"""
"""
n = int(input())
for i in range(n):
	m = int(input())
	if m % 4 == 1:
		print("ALICE")
	else:
		print("BOB")
"""
"""
n = int(input())
lst = []
for i in range(n):
	f = input()
	s = input()
	t = input()
	if  f[0] == "l" and s[0] == "l" and t[0] == "l" and t[1] == "l"     or     f[0] == "l" and s[0] == "l" and t[0] == "l" and t[1] == "l" and t[2] == "l" or f[0] == "l" and s[0] == "l" and s[1]=="l" or  f[0] == "l" and s[0] == "l" and s[1]=="l" and s[2]=="l" or f[1]=="l" and s[1] == "l" and s[2] =="l" or f[1]=="l" and s[1] == "l" and t[1] == "l" and t[2] == "l" or s[1] == "l" and t[1]=="l" and t[2] == "l" or s[0] == "l" and t[0] =="l" and t[1] == "l" or s[0] == "l" and t[0] =="l" and t[1] == "l" and t[2] == "l":
		lst.append("yes")
	else:
		lst.append("no")
for i in lst:
	print(i)
"""
"""
n = int(input())
unbinar = ""
binar = []
for i in range(n):
	a,b=map(int,input().split())
	for i in range(a,b+1):
		binar.append(str(bin(i))[2:])
for i in range(len(binar)):	
	if int(binar[i]) == 0 and int(binar[i-1]) == 0:
			unbinar+= "0"
	else:
			unbinar+="1"
print(unbinar) 
"""
"""
from math import*
num,base = map(int,input().split())
zeros = 0
newNum = ""
fnum = factorial(num)
if base < 2 or base > 10**18:
	exit()
while fnum > 0:
    newNum = str(fnum % base) + newNum
    fnum //= base
rev = newNum[::-1]
i = 0;
while 1:
	if rev[i] == "0":
		zeros+=1
	else:
		break
	i += 1
print(zeros)
print(newNum)
"""
"""
n = int(input())
lol = []
kek = n
if n > 0:
	print(n)
elif len(str(n)) == 3 and n < 0:
	print(str(n)[2])
"""
"""
c,n,f = map(int,input().split())
cows = ["*"]*c
keks = []
ind = 0
a = list(map(int,input().split()))
for i in range(f):
	p1,p2 = map(int,input().split())
	cows[p2-1] = p1
	for i in a:
		if i != p1:
			keks.append(i)
			break
	for i in range(len(keks)):
		cows[cows.index(p1)-1] = keks[i]
print(cows.index("*")+1,cows)
"""
"""
a,b = map(int,input().split())
tetr = list(map(int,input().split()))
per = []
for i in range(a):
	if tetr[i] < b:
		for i in range(tetr[i]):
			per.append(i)
			print(per)
"""
"""
a,b = map(int,input().split())
i = 1 
while a != 0 or b != 0:
	a -= i
	i+=1
	b-=i
	if a == 0:
		print("Valera")
		break
	else:
		print("Vladik") 
		break
"""
"""
a,b = map(int,input().split())
if b % a == 0 and a != b:
	print (1)
else:
	print(min(a,b))
"""
"""
a,b = map(int,input().split())
mark = list(map(int,input().split()))
suma = sum(mark)
ans = 0
if round(sum(mark) / a) == b:
	print(0)
	exit()
else:
	while round(suma / a + 0.00000000000001) != b:
		suma+=b
		a+=1
		ans += 1
print(ans)
"""
"""
n = int(input())
if n > 0:
	print(n)
	exit()
elif len(str(n)) == 3 and str(n)[2] == "0":
	print(str(n)[2])
elif len(str(n)) == 3 and int(str(n)[1]) < int(str(n)[2]) and int(str(n)[2]) != 0:
	print("-" + str(n)[1])
elif int(str(n)[-1]) > int(str(n)[-2]):
	opacha= list(str(n))
	kek = str(n).rfind(str(n)[-1])
	del(opacha[kek])
	for i in opacha:
		print(i,end="")
else:
	opacha= list(str(n))
	kek = str(n).rfind(str(n)[-2])
	del(opacha[kek])
	for i in opacha:
		print(i,end="")	
"""
"""
n = int(input())
lol = []
for i in range(n):
	a = list(map(int,input().split()))
	lol.append(sum(a))
if len(lol) == 1 and 0 in lol:
	print(1)
if len(set(lol)) == 2:
	lol.sort()
	print(lol[-1] - lol[0])
else:
	print(-1)
"""
"""
s = input()
zeros = "0000000"
ones = "1111111"
if zeros in s or ones in s:
	print("YES")
else:
	print("NO")
"""
"""
n =  int(input())
kek = set()
lol = list(kek)
for i in range(n):
    a = input()
    if a in lol:
        print("YES")
    else:
        print("NO")
        lol.append(a)
"""
"""
n = int(input())
a = list(map(int,input().split()))
kek= 0
a.sort()
for i in range(1, n, 2):
	kek += a[i] - a[i - 1]
print(kek)
"""
"""
a,b = map(int,input().split())
red  = a*2
green = a*5
blue = a*8
knig = 0
if red % b != 0:
	knig+= red // b + 1
else:
	knig += red // b
if green % b != 0:
	knig += green // b + 1
else:
	knig += green // b
if blue % b != 0:
	knig += blue // b + 1
else:
	knig += blue // b
print(knig)
"""
"""
from math import*
a,b = map(int,input().split())
primes= [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199]
bulka = True
if b - a > 1:	
	for i in range(b-a-1):
		a +=1
		if a in primes:
			opa = "NO"
			break
		elif b in primes:
			opa = "YES"
		else:
			opa = "NO"
else:
		if b in primes:
			opa = "YES"
		else:
			opa = "NO"
print(opa)
"""
"""
n = input()
ch = False
opa = 0
for i in n:
	if int(i) % 2 == 0:
		ch = True
		
if ch == False:
	print(-1)
	exit()
else:
	kek = n.replace(chlst[0],n[-1])
	print(kek)
"""
"""
n = int(input())
a = list(map(int,input().split()))
ans = 0
for i in range(n):
	kek = a.count(1)
	lol = 0
	c1 = 0
	for j in a[i:]:
		if j == 1:
			kek -= 1
		else:
			kek += 1
		lol = max(lol, kek)
		ans = max(ans, kek)
print(ans)
"""

"""
n,m = map(int,input().split(" "))
b = 1
r = 2
i = 0
if n >= m:
		while n > m:
			n -= b
			i+=1
elif  n < m:
		while n < m:
			if m % 2 == 0:
				m //= 2
			else:
				m += 1
			i+=1
		while n > m:
			n -= b
			i+=1
"""		
n = int(input())
if n % 2 == 0:
	print(n // 2)
else:
	print((n-1)//2 - n)
