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
#t = map(int,raw_input().split())

n = int(raw_input())

def prog(frist, last, n):
    return ( frist + last ) / 2 * n


if n % 2 == 0:
    end_pos = n
    end_neg = n - 1
    num_pos = n / 2
    num_neg = num_pos
else:
    end_neg = n
    end_pos = n - 1
    num_pos = n / 2
    num_neg = num_pos + 1


print prog(2,end_pos, num_pos) - prog(1,end_neg, num_neg)


##
##i = 1
##sm = 0
##
##while True:
##    t = i
##    if t % 2 != 0:
##        t *= -1
##    
##        
##    sm += t
##    if i == n:
##        break
##    i += 1
##    
##
##print sm
    
    
