#include <iostream>
 
using namespace std;
 
int main(){
  int n;
  cin >> n;
  cout << (n + 1) / 2 - 1;
  return 0;
}
### Prompt

Construct a Cpp code solution to the problem outlined:

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;
  cout << (n + 1) / 2 - 1;
  return 0;
}
```