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
#include <iostream>
using namespace std;
int main()
{
_int64 n;
while (cin >> n)
{
if (n % 2 == 0)
{
cout << n / 2 << endl;
}
else
cout << (n - 1) / 2 - n << endl;
}
}