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;
unsigned long long gcd(unsigned long long a, unsigned long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
void prt(int n) {
  if (!n) {
    return;
  }
  prt(n - 1);
  cout << n << "\n";
}
int main() {
  long long n;
  cin >> n;
  cout << (n % 2 ? "-" : "") << n / 2 + (n % 2 ? 1 : 0) << "\n";
  return 0;
}
