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
_1 = '236a'
# count = 0
# l = [list(map(int, input().split())) for i in range(5)]
# for i in range(5):
#     for j in range(5):
#         if l[i][j] == 1:
#
#             if i > 2:
#                 count += i - 2
#             elif i < 2:
#                 count += 2 - i
#             if j > 2:
#                 count += j - 2
#             elif j < 2:
#                 count += 2 - j
# print(count)
_2 = '339a'
# n = list(map(str, input().split('+')))
# n.sort()
# print('+'.join(n))
_3 = '266a'
# count = 0
# t = 0
# n1 = int(input())
# n = list(str(input()))
# for i in range(1,n1,1):
#     if n[i] == n[i-1]:
#         count +=1
# print(count)
_4 = '96a'
# flag = False
# flag1 = False
# p0=0
# p1=0
# l = list(str(input()))
# for i in l:
#     if i == '0':
#         p0 += 1
#         if p0 >= 7:
#             flag = True
#     else:
#         p0 = 0
# for i in l:
#     if i == '1':
#         p1 += 1
#         if p1 >= 7:
#             flag1 = True
#     else:
#         p1 = 0
# if flag == True or flag1 == True:
#     print("YES")
# else:
#     print("NO")
_5 = '236a'
# l = list(str(input()))
# if len(set(l)) % 2 == 0:
#     print("CHAT WITH HER!")
# else:
#     print("IGNORE HIM!")
_6 = '281a'
# l = list(str(input()))
# t = l[0].upper()
# print(t + ''.join(l[1:]))
_7 = '546a'
# s = 0
# a,b = map(int, input().split())
# while a <= b:
#     a*=3
#     b*=2
#     s+=1
# print(s)
_8 = '116a'
# t = 0
# l1 = []
# n = int(input())
# l = [list(map(int, input().split())) for i in range(n)]
# for i in range(n):
#     t += l[i][1]
#     t -= l[i][0]
#     l1.append(t)
# print(max(l1))
_9 = '977a'
# n,k = map(int, input().split())
# for i in range(k):
#     if n % 10 == 0:
#         n/=10
#     else:
#         n-=1
# print(int(n))
_10 = '266b'
# n = str(input())
# l = list(n)
# low = 0
# up = 0
# for i in l:
#     if i.islower():
#         low += 1
#     else:
#         up += 1
# if low >up:
#     print(n.lower())
# elif low < up:
#     print(n.upper())
# else:
#     print(n.lower())
_11 = '110a'
# n = list(str(input()))
# l=[]
# for i in range(len(n)):
#     if n[i] == '4' or n[i] == '7':
#         l.append(i)
# if len(l) == 4 or len(l) == 7:
#     print('YES')
# else:
#     print('NO')
_12 = '41a'
# n = str(input())
# n1 = str(input())
# if n1 == n[::-1]:
#     print('YES')
# else:
#     print("NO")
_13 = '734a'
# k = int(input())
# n = list(str(input()))
# a =0
# d =0
# for i in n:
#     if i == 'A':
#         a += 1
#     else:
#         d += 1
# if a >d:
#     print('Anton')
# elif d > a:
#     print('Danik')
# elif a == d:
#     print('Friendship')
_14 = '133a'
# n = list(str(input()))
# flag = False
# for i in n:
#     if i == 'H' or i == 'Q' or i == '9':
#         flag = True
# if flag:
#     print('YES')
# else:
#     print('NO')
_15 = '467a'
# count = 0
# n = int(input())
# l = [list(map(int, input().split())) for i in range(n)]
# for i in range(len(l)):
#     if l[i][1] - l[i][0] >= 2:
#         count += 1
# print(count)
_16 = '158a'
# count = 0
# flag = False
# n,k = map(int,input().split())
# l = list(map(int, input().split()))
# ball = l[k-1]
# for i in l:
#     if ball > 0:
#         if i >= ball:
#             count += 1
#     else:
#         flag = True
# if flag:
#     for i in l:
#         if i > 0:
#             count += 1
#     print(count)
# else:
#     print(count)
_17 = '677a'
# count = 0
# n,h = map(int, input().split())
# l = list(map(int, input().split()))
# for i in l:
#     if i > h:
#         count += 2
#     else:
#         count += 1
# print(count)
_18 = '580a'
# n = int(input())
# l = list(map(int, input().split()))
# r = []
# count = 0
# for i in range(0,n):
#     if l[i-1] <= l[i]:
#         count += 1
#         r.append(count)
#     if l[i-1] > l[i]:
#         count = 0
#         count +=1
# print(max(r))
_19 = '344a'
# n = int(input())
# l = [list(map(int, input().split())) for i in range(n)]
# c = 1
# t = l[0]
# for i in range(1,n):
#     if l[i] != t:
#         c+=1
#         t = l[i]
# print(c)
_20 = '1030a'
# flag = False
# n = int(input())
# l = list(map(int, input().split()))
# for i in l:
#     if i == 1:
#         flag = True
# if flag:
#     print('HARD')
# else:
#     print('EASY')
_21 = '705a'
# n = int(input())
# n1 = 0
# if n%2==0:
#     n1 = n/2-1
#     n1 = int(n1)
# else:
#     n1 = ((n+1)/2)-1
#     n1 = int(n1)
# h = 'I hate '
# t = 'that '
# l = 'I love '
# if n == 1:
#     print('I hate it')
# elif n == 2:
#     print('I hate that I love it')
# elif n == 3:
#     print('I hate that I love that I hate it')
# elif n > 3:
#     if n % 2 == 0:
#         print(((h+t)+(l+t))*n1+((h+t)+(l+'it')))
#     else:
#         print(((h+t)+(l+t))*n1+(h+'it'))
_22 = '318a'
# n,k = map(int, input().split())
# l = []
# for i in range(1,n+1):
#     if i%2!=0:
#         l.append(i)
# for i in range(1,n+1):
#     if i%2==0:
#         l.append(i)
# print(l[k-1])
_23 = '479a'
# n1 = int(input())
# n2 = int(input())
# n3 = int(input())
# ans = n1+n2+n3
# ans = max(ans,(n1+n2)*n3)
# ans = max(ans, n1*(n2+n3))
# ans = max(ans, n1*n2*n3)
# print(ans)
_24 = '486a'
n = int(input())
if n%2 ==0:
    print(int(n/2))
else:
    print((round((n+1)/2))*(-1))
