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
import collections

def getDP(table, *ind):
    for t in ind:
        if t not in table:
            print("non presente")
            return None
        table = table[t]
    return table

def setDP(table, val, *ind):
    for t in ind[0:-1]:
        if t not in table:
            table[t] = {}
        table = table[t]
    table[ind[-1]] = val
#####################################

n = int(input())
if n % 2:
    print(int(-(n+1)/2))
else:
    print(int((n+1)/2))
