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
#_____________75____________#
# n,k=map(int,input().split())
# half=(n+1)//2
# print(2*(k-half) if k>half else (2*(k))-1)
    
#_____________76____________#
# n=int(input())
# li1=[int(x) for x in input().split()]
# li2=[int(x) for x in input().split()]
# print('I become the guy.' if len(set(li1+li2).difference([0]))>=n else 'Oh, my keyboard!')

#_____________77____________#
# import math
# d,n=map(int,input().split())    
# print(-1 if d==1 and n==10 else int((n)*math.ceil((10**(d-1))/n)))

#_____________78____________#
# n=int(input())
# li=[]
# for i in range(n):
#     li.append(input())
# teams=list(set(li))
# if len(teams)==1:
#     print(teams[0])
# else:
#     print(teams[0] if li.count(teams[0])>li.count(teams[1]) else  teams[1])

#_____________81____________#
# txt=input().split('WUB')
# print(*[i for i in txt if i])

#_____________82____________#
# n=int(input())
# vec=[]
# for i in range(n):
#     vec.append(input())
# x=True
# r=c1=0
# c2=n-1
# letter=vec[0][0]
# while r<n:
#     if vec[r][c1]!=letter or vec[r][c2]!=letter:
#         x=False
#         break
#     c1+=1
#     c2-=1
#     r+=1
# if x:
#     line=''.join(vec)
#     letters=set([i for i in line])
#     # print(letters)
#     if len(letters)!=2 or line.count(letter)!=(n*2)-1: x=False
# print('YES' if x else 'NO')    

#_____________84____________#
n=int(input())
print(n//2 if n%2==0 else -(n+1)//2)