#include <bits/stdc++.h>
int main() {
  int x = -1;
  unsigned long long int a, i;
  scanf("%llu", &a);
  long long int toplam = 0;
  if (a % 2 == 0)
    printf("%llu", a / 2);
  else
    printf("%lld", (a + 1) / 2 * -1);
  return 0;
}

### Prompt

Please formulate a Cpp solution to the following problem:

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

### Response

```cpp
#include <bits/stdc++.h>
int main() {
  int x = -1;
  unsigned long long int a, i;
  scanf("%llu", &a);
  long long int toplam = 0;
  if (a % 2 == 0)
    printf("%llu", a / 2);
  else
    printf("%lld", (a + 1) / 2 * -1);
  return 0;
}

```