#include <bits/stdc++.h>
using namespace std;

int main() {
  int n;
  cin >> n;
  cout << (n-1-(n+1)%2)/2 << endl;
  return 0;
}
### Prompt

Generate a cpp solution 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 <bits/stdc++.h>
using namespace std;

int main() {
  int n;
  cin >> n;
  cout << (n-1-(n+1)%2)/2 << endl;
  return 0;
}
```