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())
pre='00'
c=0
for i in range(n):
    nn=input()
    if pre!=nn:
        c+=1
        pre=nn
print(c)
'''
'''
t=int(input())
for i in range(t):
    n,k=map(int,input().split())
    td=0
    for j in range(n):
        t1,d1=map(int,input().split())
        if t1 > k:
            td+= (t1-k) * d1
            #k = 0
        else:
            k = k - t1
    print(td)
'''
'''
n=int(input())
for i in range(1,n):
	if(i%2==1):
		print("I hate that",end=" ")
	else:
		print("I love that",end=' ')
if n%2==1:
	print("I hate it",end=' ')
else:
	print("I love it",end=' ')
'''
'''
n=int(input())
ar=list(map(int,input().split()))
a=sum(ar)
if a:print('HARD')
else:print('EASY')
    
'''
'''
n=int(input())
c=1
m=int(input())
for i in range(n-1):
    nextn=int(input())
    if nextn!=m:
        c+=1
        m=nextn
    else:
        continue
print(c)
'''
from math import ceil

n = int(input())

print(ceil(n/2)*((-1) ** n))
