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 timeit
#start=timeit.default_timer()
n = int(input())

if n%2==0:
    i0=2
    iN=n
    sumi=int(int(int(i0+iN)/2)*int(int(int(iN-i0)/2)+1))
    k0=1
    kN=n-1
    sumk=int(int(int(k0+kN)/2)*int(int(int(kN-k0)/2)+1))
    print(sumi-sumk)
else:
    i0=1
    iN=n
    sumi=int(int(int(i0+iN)/2)*int(int(int(iN-i0)/2)+1))
    k0=2
    kN=n-1
    sumk=int(int(int(k0+kN)/2)*int(int(int(kN-k0)/2)+1))
    print(sumk-sumi)

#stop=timeit.default_timer()

#print("Time : "+str(stop-start))