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
# ll soma(ll a1, ll an, ll n)
# {
#     return ((a1+an)*n)/2;
# }


def soma(a1,an, n):
    n =int(n)
    a1=int(a1)
    an=int(an)
    return int(int(int(a1+an)*n)/2)

    # int n;
    # cin >> n;
    # ll res=0;
    # if (n%2==0)
    # {
    #     res = soma(2,n, n/2);
    #     res -= soma(1, n-1, n/2);
    #     printf("%lld\n",res );
    # }
    # else
    # {
    #     res = soma(2,n-1, n/2);
    #     res -= soma(1, n, (n+1)/2);

    #     printf("%lld\n",res );
    # }

n = int(input())
if (n%2==0):
    res = soma(2,n, n/2);
    res -= soma(1, n-1, n/2);
    print(res)
else:
    res = soma(2,n-1, n/2)
    # print (res)
    res -= soma(1, n, (n+1)/2)
    print( res)
