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 <bits/stdc++.h>
using namespace std;
int main() {
  long long int n;
  cin >> n;
  if (n % 2 == 0)
    cout << n / 2 << endl;
  else
    cout << -1 * (n / 2 + 1) << endl;
  return 0;
}
