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

In cpp, your task is to solve 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;
  if(n&1)
  {
    n++;
  }
  cout<<n/2-1;
}
```