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