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
withFile = 0

if(withFile == 1):
    fin     = open('input.txt', 'r')
    fout    = open('output.txt', 'w')

def getl():
    if(withFile == 0):
        return input()
    else:
        return fin.readline()
def printl(s):
    if(withFile == 0):
        print(s)
    else:
        fout.write(str(s))
def get_arr():
    x = getl().split(' ')
    if(x[-1] == ''):
        x = x[:-1]
    return list(map(int, x))

n = get_arr()[0]

if(n % 2):
    print(int(int(n-1) / 2) - n)
else:
    printl(int(n / 2))



if(withFile == 1):
    fin.close()
    fout.close()

