#include <iostream>
using namespace std;
int main() {
  int N;
  cin >> N;
  int ans = (N - 1) / 2;
  cout << ans << endl;
}

### Prompt

Please create a solution in cpp to the following problem:

How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

* 1 \leq N \leq 10^6
* N is an integer.

Input

Input is given from Standard Input in the following format:


N


Output

Print the answer.

Examples

Input

4


Output

1


Input

999999


Output

499999

### Response

```cpp
#include <iostream>
using namespace std;
int main() {
  int N;
  cin >> N;
  int ans = (N - 1) / 2;
  cout << ans << endl;
}

```