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
# n=int(input())
# if(n%2==0):
#     print("YES")
# else:
#     print("NO")
# for _ in range(int(input())):
#     n=(input())
#     if(len(n)<=10):
#         print(n)
#     else:
#         print(n[0]+str(len(n)-2)+n[len(n)-1])
# a=0
# for _ in range(int(input())):
#     n=list(map(int,input().split()))
#     count=0
    
#     for i in range(len(n)):
#         if(n[i]==1):
#             count+=1
#         else:
#             count-=1
#     if(count>0):
#         a+=1
# print(a)
# n,m=map(int,input().split())
# a=list(map(int,input().split()))
# count=0
# for i in range(len(a)):
#     if(a[i]>=a[m-1] and a[i]>0):
#         count+=1
# print(count)
# n,m=map(int,input().split())
# # if((n*m)%2!=0):
# print((n*m)//2)
# # else:
# #     print((n*m)//2)\
# x=0
# for _ in range(int(input())):
#     n=input()
#     if(n=="X++" or n=="++X"):
#         x=x+1
#     else:
#         x=x-1
# print(x)
# n = input()
# m = input()
# n = n.lower()
# m = m.lower()
# if n == m:
#     print("0")
# elif n > m:
#     print('1')
# elif n <m:
#     print('-1')
# matrix=[]
# min=[]
# one_line=0
# one_column=0
# for l in range(0,5):
#     m=input().split()
#     for col,ele in enumerate()

# a = list(map(int,input().split('+')))
# a.sort()
# print('+'.join([str(c) for c in a]))
# n=list(input())
# # if(n[0].islower()):
#     n[0]=n[0].upper()
# else:
#     pass
# print("".join(str(x)for x in n))
# n=list(input())
# s=input()
# count=0
# for i in range(1,len(s)):
#     if(s[i]==s[i-1]):
#         count+=1
# print(count)
# v=["A","O","Y","E","U","I","a","i","e","o","u","y"]
# n=list(input())
# x=[]
# for i in range(len(n)):
#     if n[i] not in v:
#         x.append(n[i])
# print("."+".".join(str(y.lower())for y in x))
# a=[]
# b=[]
# c=[]
# for _ in range(int(input())):
#     x,y,z=map(int,input().split())
#     a.append(x)
#     b.append(y)
#     c.append(z)
# print("YES" if sum(a)==sum(b)==sum(c)== 0 else "NO") 
# m = "hello"
# n=input()
# j=0
# flag=0
# for i in range(len(n)):
#     if(n[i]==m[j]):
#         j=j+1
#     if(j==5):
#         flag=1
#         break
# if(flag==1):
#     print("YES")
# else:
#     print("NO")
# a=set(list(input()))
# print("CHAT WITH HER!" if len(set(list(input())))%2==0 else "IGNORE HIM!")
# k,n,w=map(int,input().split())
# sum=0
# a=[]
# for i in range(w+1):
#     sum+=k*i
# print((sum-n) if sum>n else 0)
# m,n = 0,0
# for i in range(5):
# 	a = map(int,input().split())
# 	for j in range(5):
# 		if a[j]!=0:
# 			m = i
# 			n = j
# 			break
# print(abs(m-2)+abs(n-2))
# l,b=map(int,input().split())
# c=0
# while(l<=b):
# 	l=l*3
# 	b=b*2
# 	c=c+1
# print(c)
# from math import ceil 
# n,m,a=map(int,input().split()) 
# # print(ceil(n/a),ceil(m/a))
# c=ceil(n/a)*ceil(m/a)  
# print(c) 
# n=int(input())
# if(n%4==0 or n%7==0 or n%44==0 or n%47==0 or n%74==0 or n%444==0 or n%447==0 or n%474==0 or n%477==0):
# 	print("YES")
# else:
# 	print("NO")
# def tramCapacity():
#     n = int(input().strip())
#     pout, pin = map(int, input().strip().split())
#     sm = pin
#     mx = pin
#     for i in range(n-1):
#         pout, pin = map(int, input().strip().split())
#         sm = sm - pout + pin
#         if sm > mx:
#             mx = sm
#     return mx

# print(tramCapacity())
# n,k=map(int,input().split())
# for i in range(k):
# 	if(str(n)[-1]=="0"):
# 		n=n//10
# 	else:
# 		n=n-1
# print(n)
# n=int(input())
# n=int(input())
# if(n%5==0):
# 	print(n//5)
# else:
# 	print((n//5)+1)
n=int(input())
if(n%2==0):
    print(n//2)
else:
    print("-"+str(n-((n-1)//2)))